For me, it boils down to how thick the wood is and how tight do I need to make my turns. I generally don't use anything bigger than #5 because I'm not cutting anything really thick (usually 1/4" to 1/2"). If I'm cutting 1/8", I'll go all the way down to a 2/0. I'll use a number 7 blade if I have to make long straight cuts, because a larger blade will track better on a straight line.
As for hole drilling for veining cuts (highlights), it depends on the size of the blade I'm using. For any application like this, use the smallest bit the blade will fit through. If there's waste material coming out, I use a larger bit to make it easier to thread the blade through (especially since I'm a bottom feeder!
)
This is one of those areas where experience and personal preference reign! Do what works best for you!
My 2 cents. I'm sure others will chime in!
Good Luck, MW