Hello Bob,
There is no real “rule of thumb”, as long as the bit is either the exact size you want your hole or smaller you are fine.
Only difference is if you use the exact size bit you can use the “drill function” in a lot of the software options. Vs if it is smaller you would use the “pocket” cut to achieve your end result. But either way you can achieve good results.
Nice thing about the drill feature (if you happen to have the right size endmill for the job) is it takes bites up/down so it clears out the hole as it drills. The pocket function just progressively goes deeper until it recaches the depth you set. But again I use the pocket feature all the time and have great results.
-Alex