As above.
It's actually important to continually rotate your tyres so that the spare wears at the same rate to avoid problems. I have first-hand experience with this problem and haven't been rotating tyres - so with the lesson learned, I thought I should pass it along.
I blew one of my 255/70R16 BFG ATs a couple of weeks ago, and put my steel-rimmed spare - an original Continental 255/70R16. Same tyre size, but the Conti had only ever seen about 100km on the road so it had a greater tread depth - and thus a larger rolling diameter. To avoid problems, I made sure this tyre went on a front wheel.
Well, it caused me problems. I engaged 4WD when I needed it (flooded road) and it wouldn't disengage - the transmission started to bind up. I had to make some left-hand turns (the Conti was on the left front) so that the right-hand wheel could "catch up" before 4WD disengaged.
Moral of the story (that I've now learned the hard way) - rotate your tyres and NEVER use an unmatched tyre unless it's an emergency - and if you do, put it on the front and try to avoid 4WD.
Yes, I'm shopping for 5 tyres now. These BFGs have seen 80,000km and it's time to get some new shoes. I'm thinking BFGs have been good to me, but am happy to consider Goodyear Wranglers or Mickey Thompson ATs - I won't ever touch Bridgestone, and don't like the reports of chipping people have made about Cooper tires, although some people swear by them.
