I used to level the reticle as you do by shouldering, adjusting, shouldering, adjusting until it looked OK and lock it down but several times I would get to the range and bag it on the bench and the reticle would be obviously rotated. PITA! Also I mount many of my friend's scopes and they used to complain about the reticle being rotated.
Now I don't have that problem anymore and after mounting a scope, it always seems level when I look through it standing offhand or on the bench. Plus I know that the reticle is perfectly level.
This is the way I do them all now:
First I have an EXD ENGINEERING : EXD ENGINEERING VERTICAL RETICLE INSTRUMENT - World's Largest Supplier of Firearm Accessories, Gun Parts and Gunsmithing Tools
which is perfectly made to be level
Set your rifle in a vise and aim it out the door or window to a distant level object (I use a 4' level on a fence 30 yards out of the door). The EXD fits on the tube of the scope and the barrel of your gun
rotate your rifle until the EXD is level and clamp the rifle down. This will align your scope tube and bore in a vertical line and take all the cant out of your rifle. While it is uncanted, aim at the distant horizontal or vertical object and rotate your scope until the reticle is aligned with that distant object.
You will find that the real trick is to tighten the rings down and have the EXD level and the reticle aligned after tightening. If you have the kind of rings that have a top half then it is not bad, you just have to alternate tightening the screws. If you have the kind that tighten horizontally at the top then you may have to start with the bubble a little to one side and it will end up level after tightening.
Anyway, after you have created a level reticle when your rifle is uncanted, you will need to duplicate that in the field or on the bench. This calls for an anti-cant device. The best one I have found it the Scoplevel Anti Cant Leveling Device
It folds down out of the way but when you need it you can fold it up and see it without taking you cheek off the butt of the stock.
Does it help? I believe it has reduced my group size at 300 yards or more but this variable is kinda mixed in with all the other variables like wind, loads, rifle etc. so it is hard to quantify.
Has it removed the feeling of not having a level reticle? Absolutely.