Depends on what kind of chamber you have and level of accuracy you are trying to achieve. Most factory chambers are so oversize that lot of extra work equals little benefit which is what green788 is talking about.
However, if you have custom match chamber, then it can make all the difference in the world by most people who shoot extreme accuracy.
Trimming- most people trim to what it says in the book. However, they have no idea what lenght their chamber neck actually is. www.sinclairintl.com
has chamber length guages (p 34)to put in a case to measure actual neck length. You might want to trim to closer (.010) to max to limit carbon build up.
What is the advantage some will say? If you trim to book measurement and your chamber is .030 longer, you have an area for carbon build up which is detrimental to extreme accuracy. Most people would be shocked at what their chamber throat and bore actually looked like. Carbon is harder to get out than copper and it actually layers carbon, copper, carbon etc.
If you are going to have a gun built have your smith make a "bump guage" it is nothing more than a barrel stub reamed to fit on the shoulder. Cost is about $30. Easy to measure shoulder movement. Stoney point sells a set of headspace guage tools(35.65) from sinclair again.
I normally use the Redding comp shellholders which have .002 increments. Never have to touch my dies once set up and just change shellholder if I want a bump.
Seating bullets- couple general rules which can always be broken. Magazines you are limited to mag OAL. However, if you want to fire singleshot, then VLDs generally like some jam (ie as much as .030 into lands) while bullets like Sierras generally like it off the lands. Mag guns are limited to OAL which can lead to inconsistencies in ogive to lands. In single shot OAL is meaningless, you want to measure where ogive is to the lands.
How to find the lands.
Slight neck tension seat a bullet long and "crush" seat by loading into chamber. do a couple times and that will generally give you about .050 into lands. More accurate is the sinclair seating depth tool ($28.95). Very easy to measure and monitor throat erosion. Mags throats can move as much as .003 or more per 100 rds.
After that I start load development at either max mag oal and come back or at about .030 in lands and shoot a ladder.
I strongly differ with Green788 on advantage of the OCW vs ladder particularily in custom guns. Ladder is proven time and time again in extreme accuracy guns. Have not heard of even one top notch competitor said they developed their load on OCW, until that happens, OCW is at best a good idea for mid level accuracy if that is all you want IMO, but that is a whole different arguement.
I use a chrono and find the barrel nodes for that load. one 20-25 shot ladder, I have confirmed tuning nodes (2-3 normally), and I can tune in the middle of one of the nodes to avoid temp related pressure spikes, know where my pressure limts are,know if the powder is going to work and within .3 gr of final load, and then in about 2 more quick sessions I can fine tune the load, seating depth and neck tension.
I have taken new barrels and guns from the smith to competition and won or placed very high in as little as 3 days of short evening testing and only 65 rds, so I know dang good and well the ladder method works for extreme accuracy.
Testing without a chrono is a waste of bullets, barrels, powder and time IMO if you are after serious accuracy which is another difference with OCW and ladder.
Primer pockets and flash holes. I do not touch flashholes that are drilled (right now Lapua). I normally ream the others except for BR cases, they like the tighter holes. Test 5 reamed cases vs 5 unreamed and you will need a chrono. You might find that reaming them opens up your ES and SD. I always go for lower ES, if you have low ES you will automatically have low SD.
Primer pockets- better brass is more uniform and benefit is more limited. However, all top notch competitors normally do it. Why, because they have proven to themselves it works in custom guns with extreme accuracy levels.
Anyone can do a simple test: uniform a bunch of primer pockets, look at the amount of material taken off one side or the other and say does common sense say that might make a difference IF I did not do it for extreme accuracy. Will that primer fire if setting in lopsided, will it effect accuracy, then use your common sense. Why would anyone build a $3000 gun and not do it? $20 B&D power screwdriver with uniformer and 10 minutes does 100 pieces of brass while watching TV in the winter.
If I have a factory chamber and 1 MOA accuracy standard, then probably no benefit. Some are anal and weigh primers. Why, spots discrepancies. Federal had a bad lot of F210 primers get out two years ago and their quality control was non-existant in that lot. It was proven that the ES using those primers went into almost triple digits, go to another lot and back down to single digit. You could visually see in most cases and could measure differences in weight. I normally do not weigh, but I either use F210, RWS or CCI BR primers. Only use mag primers on the 338 Lapua Ack Improved.
Powders- I always go with powders that are not temp sensitive. I always record my lot #. Why, wide lot to lot variation, for example 4 yrs ago Alliant let out a lot of RL 22 that was extremely hot. I loaded my normal load (did not test) went to National Comp and was blowing primers, and broke 3 triggers from blowback in 2 days before I quit trying to use it. I had to drop 4 grains from my previous load. Always record and always test new lots. When you find one you like, buy lots of it. I have 30 lbs left of my favorite comp powder which is Norma MRP.
Bullets- I sort by weight and base to ogive. Weight for my standards is +- .001. I measure ogives with the Buhay ogive checker ($125) from RW Hart and David Tubbs sells the same checker under his name. It is very fast, easy and give uniform results. My last box of 338 SMKs had an .016 extreme spread with over 480 falling into .005. You can use a pair of Stoney Pt bullet comparators on a dial caliber. They are cheaper, slower, subject to variations in how you measure but workable.
Cleaning- more thoughts than you care to hear. However, couple principles always apply.
1. good bore guide if you like your throat and chamber. Lucas is the best $40
2. Good rods, I like Dewey coated rods and if long barrels, Dewey will put a 10" SS extension on it so it is 50". Boretech has good bearings in the handles but rod coating peels off and warranty service with them is pain in the ___.
3. cleaning chemicals require you to remove two things. Carbon and copper. For copper I like Montana Extreme 50 BMG also works good on carbon. Proven with borescopes. Google "precision shooting magazine" Jan 06 issue and good article on the best carbon cutter out there, I believe the name was Slip200TM (think that was the name). They also like Bore Tech Elimator for copper, but I tried it two years ago and 50 BMG beat the socks off the Eliminator.
I use hard bristle brushes from Kleen Bore or Montana Extreme. Do not use bronze anymore normally.
More than enought food for thought and contraversy.