Re: When do you bed??
A proper bedding job can only help.
With a well engineered/made stock such as McMillan, it's not likely to make a huge improvement in your accuracy.
But, it will help ensure that you get consistent/repeatable results over time and with disassembly, cleaning, and reassembly.
With laminate or wood stocks, it can make a huge difference over time and varying weather conditions.
And, if you don't do it from the start, then you run the risk of having to start over with your load development.
There are lots of good videos and explanations here. But, you can only learn by doing it. I would practice on a less expensive stock first if possible.