On my DH bike I do run a giant tire in the front and a slightly smaller model in the back. But generally off-road I'd just make sure that the tire with more traction potential is in the front. If the front tire is wider and bald and the back tire is narrower and knobby, you are still going to lose the front end first in a corner.

On the road, the traction differences between different thicknesses of tires has never been noticible. I have occasionally run a 25c in the back and a 23c in front. Bigger differences would probably not cause any real problems.