depends
The word "depends" appears several times in previous responses, for good reason.
With the newer kits there may (or may not) be less ambiguity but I'm sure it still depends a lot on factors specific to your particular build. For me it was hard to know for sure until I had the engine hung, and all of the instruments at least temporarily in the panel, and most of the harnesses at least partly done.
My point being, delay as long as possible then use stiff wire / string / etc. to simulate the routing of all items and sort out any unexpected interference etc. before committing to the holes and pass-thrus. And pay attention to where you'll need to get your hands in there past accessories, or to remove the oil filter, or to remove the magneto, etc. so you don't end up with a wire bundle in the way of something like that.
EDIT: I see I may be in disagreement with Bob who says "it will never be easier to drill the stainless steel firewall than it is right now". Which is absolutely correct, and if you can know with confidence where there could be interference from engine accessories beforehand (like, by suspending the engine in place temporarily) then his may well be the better approach.