Given the choice, I'll always pick the right size inner tube for my tire. As simple as that sounds, that's getting progressively harder to do because there are so many slightly different tire sizes and valve length configurations today that many local bike shops don't stock them all.

Whenever I place a smallish internet order, I'll frequently maximize my shipping benefit by adding a few inner tubes of the exact sizes to match the bikes that Mrs. Grouch and I are currently using. If I'm forced to pick an inner tube that isn't an exact match, I prefer to use one that's a little too small over one that's a little too big. In a real pinch I've used some pretty mismatched tire and tube combinations including using a 26" tube in a 20" tire (you telescope the tube inside of itself).