Least worn tyre: front or back? Where should I put the least worn tyre? on the front or the back? This is for XC MTB.
The Good or less worn tire should go on the front wheel as indicated in this Sheldon Brown Article on Tire Rotation
Although the article does hint to the fact that you should not be really rotating tires to the front wheel that may have been used on the rear wheel.

The only time tire rotation is appropriate on a bicycle is when you are replacing the rear tire. If you feel like taking the trouble, and use the same type of tire front and rear, you should move the front tire to the rear wheel, and install the new tire in front.
The reason for this is that the front tire is much more critical for safety than the rear, so you should have the more reliable tire on the front.
If you have a blowout, if it is on the rear tire, you have a very good chance of bringing the bike to a controlled stop. If your front tire blows, you can lose steering control, and a crash is a real possibility.