I never heard of hardwood dowels causing splitting of a buttstock when used to plug screw holes in this manner. You shouldn't have anything to worry about.

I have seen stocks that split when lead shot is stuffed into a hole in the butt in order to add weight. Over time, the lead shot oxidizes and swells, eventually splitting the wood.

Originally Posted by Geoff Roznak
The NRA has proven itself unreliable and corrupt.
Period.