The importance of a proper fitting thumb hole

[Hammer]

Getting the thumbhole to fit right is important but getting the fingerholes and the span is very important also. It is kind of hard to bowl with a ball with fingerholes drilled for a lefty if you are right handed. It will effect your game and might give you fingertip problems.

[Rdmonster]

After having thumb issues for a while and finally getting it figured out I agree that the thumbhole is number one in importance for a good release and a freer arm swing.