kr32: I would not suggest keeping a tire that is WORN OUT. I would use one from the rear that has worn the center of the tread flat to about 1/4in as a spare. It makes it easier to fold, is slightly lighter, and has shown that it is reliable up to this point. On long tours (where I would be most likely to carry a spare) this tire could safely (IMHO) be used on front OR rear.

The 1/4in I am talking about is that flattened center of tread where the 'casting flash' starts life. A narrow band will start to appear when the tread starts to wear. When this band gets to be about 1/4in wide, I start moving tires around.

If someone cuts a sidewall badly, or has a blowout, they would be glad they had a spare.