Although I think the index finger rule is excessive (according to that I have to buy some Ivo-size grips, and I have medium-large size hands), going too small is dangerous. Until recently I was playing with 4 3/8, and while it was fine when I was smaller, as I grew I started to get wrist pain. It got pretty severe, but since then I've built up the grip and I'm wrapping my wrist.
Through this, my volleys have improved astronomically. The extra stability is priceless, and I wouldn't trade it for any amount of "whippage."
I also feel that with a larger grip I can place my serve better, but other people probably have different experiences.
Ignore List: Japanese Maple, bet