A spare will age out as well as the driven tires will. Unless they have a directional tread, you're better off to use a 5 tire rotation scheme. That will also add 20% to the life of your tires so you're not really out anything. It just costs more at longer intervals.
It's also a good way to make sure your spare tire carrier works right.
I bought my '08 pickup 4 years ago and it also has the original spare that's never been used. I'll need new tires next year and will likely replace all 5 and go to the 5 tire rotation.
