Oh, you mean inner tubes right. I was thinking frame tubes for some reason, which would make your post...odd.
Anyways, smaller diameter tubes can work with larger tires, in my experience, as long as pressure is high enough, which it usually is. Let's you're using a tube rated for 1.8-2" on a 2.2" tire. Below...20 PSI you might have problems. Which is pinch flat territory anyways. If you've ever inflated a tube outside of a tire you know that they expand pretty easily.
Larger tubes in smaller tires I do not reccomend. Maybe if it's really close.
Note that I'm speaking only from my own experience. Different brands of tubes and tires act a little differently, but if you're really all that concerned, just buy a new tube. They're 5 bucks.