Depending on your shooting rig, you'd probably be off by an inch or two at best. The absolute best thing to do -- and one you really need to do anyway -- is to step off 10 yards from the target and see for yourself what your POI (point of impact) is by putting your 20-yard pin right on the bullseye at 10 yards. That way, you'll know how to compensate.

Don't feel bad though that you've never thought about it (at least your post sounded like you haven't done it yet; otherwise, there'd be no need to ask the question )... Most guys don't know they need to use their 50- or 60-yard pin at 3 yards, either!