i think it's eaisier to "up-tube" than to "down-tube", meaning a tube designed for a narrow tire in a wider tire works better than a tube designed for a wide tire in a narrower tire. now, as long as the widths are relatively close, anything should work as long as your diameters match up, but trying to put a tube designed for a 38 tire in a 23 tire might present some problems when trying to mount the tube and tire on the rim; you'll have far more excess tube than you want.