I have heard often on here that forward pitch is the way to go. I believe my pitch is probably about zero. My question is, can you fill your thumb hole with tape, and not have to grip? I currently have a fair amount of tape in my hole, and I can put my thumb in the hole. The ball doesn't fall off my hand when I hold it at my side, however it does slip down to my knuckle and hang there. This is uncomfortable when I am going through my approach. Is a possible answer to this problem the additon of more tape, or is forward pitch definately the way to go? As it stands, I have to grip the ball slightly so I don't have it feel uncomfortable through the approach.

if youre having to grip the ball during your approach, your thumb fit is improper. have someone at your Pro Shop adjust it. most people in my opinion can hold on to a ball quite readily with 0* of thumb pitch if the fit is good.
you may be one of the many bowlers suffering with "flat thumb in round hole" disease, and you may need to have your thumb refit as an oval. probably one thats flatter than you think.
if you have all of the tape at the front or back (or both) of the thumb hole, you are essentially making your own custom oval. i see this a lot at the shop, and when im drilling for people at the shop who already have i'll fitting thumb holes that contain lots of tape, they always say the same thing "this ball fits great but it doesnt always come off of my thumb." i would say that this inconsistency is due to the variable nature of tape.. and the more of it you have stacked the more variable that nature becomes.

what i like to do when people come into my shop with this problem (especially when they have thumb slugs) is measure the hole size with the tape in the ball, then measure the custom "oval" they have created with it. then i go down from that measurement by 2 bit sizes and cut the new slug to this size with an oval. this way, i know i will have to enlarge the thumb slightly and this gives me plenty of material to work with to do so.

this procedure has worked every time so far, and has actually turned a couple of "mad tapers" who thought they had some kind of nebulous swelling thumb disease into bowlers who no longer need ANY tape at all... even after 3 games.

To add to what Atomic said, after having your thumb hole fit your thumb properly and if your thumb still slips out of the ball try using as little bevel as possible, if it still slips you need to start going forward a 1/4" at a time until you hang in the thumb, then back off 1/8". Before going forward, your thumb must fit your thumb hole and the span must be fit properly or you could injure yourself. You can go forward as much as you want, but if the thumb doesn't fit the hole then the ball will still fall off your hand.