IMO, and that of both the tyre shops I use; you should rotate the spare into the mix if you do a lot of 4WDing. The reason for this is that you could have up to 15mm difference either from left to right and front to rear, this is bad for a transfer case just like running a pair of 32" up front and 31" at the back.

Also, rotating tyres does increase their life. In fact, if you don't rotate them some tyre manufacturers refuse to honour warranty claims. I do it almost every time I go on a decent 4WD track cause I always get a wheel alignment done then. It is recommended that you get the alignment done after an offroad stint as the extra stress on the running gear will throw the alignment out.

Brett