Guitar tops (usually of spruce) are usually the problem and most vulnerable to splitting. Back and sides (best wood for that is rosewood, now uncommonly used) seldom are as vulnerable. Prime tonewoods are quartersawed, book matched and old growth. Spruce top of course must be braced as it carries the bridge and force of string tension. So your view is that varnishing the underside of the spruce top would not significantly improve its resistance to shrinkage?