Have used bike inner tube on several bows, still have some on one of my longbows. It works ok, but the rubber 'ShurGrip' sleeve available from trad archery suppliers (including Black Widow) is MUCH better. Its thinner and pretty flexible so rolls up and down the grip easier than tube, and it hugs the riser tightly for a better 'form fit'. Durable, and good for bows with slippery grips, in cold weather, two-piece takedowns (hides and protects the seam), to protect leather grips from weather, or just to make a more comfortable hand fit. Two of my longbows (one with a connexxion hinge) have them...I like the solid, non-slip feel they give.
If you want a rubber grip but don't shoot your bow much the bike tube will work ok, but if you shoot it alot check out the ShurGrip at about $7 (not exactly a bank-breaker).