I've been playing almost a year, smallish hands. I dont quite hold "baseball bat", yet dont support solely by the thumb. The flat of my thumb AND the base knuckle of my index finger (side) form a V on the neck. I find I have zero control touching the neck with only my thumb.

I can play enough to have fun, but do I have a bad habit that I should be working over? What are the negative results of supporting the fretting hand with the index finger base?