which tire goes on the front
 

when mtn biking does the fast rolling tire go on the front or the back when you have a traction tire on the other wheel?

I think it would be hard to say with knowing what the tires are, exactly. And it depends somewhat upon what you mean by 'traction' and 'fast rolling'. Many tires combine both attributes to one degree or another.

This would be an excellent question to also post over at the MTB sub-forum.

Traction on the front, smooth tire (fast rolling) on the rear.
Even tho your rear might skid a little in gravel, you'll want to most of the traction up front so you can have good steering control.

Yep

It depends.

It depends on riding style and relative aggressiveness, desired results, bike design, the local terrain and the types of tires involved. Much more information is needed before any sort of applicable advice could be given.

Think of it this way: On which end would you more easily tolerate a loss of traction?

A bigger, knobbier tire up front works good for traction. If your installing a new knobby tire, put it on the front.