I thought long and hard over the same issue, I stayed stock size partly over the spare, and partly because my X does not need to be my primary off road vehicle. YMMV.
In the process though I asked my tire shop about getting me a used spare, they were very open to the idea, but the 255/85/16's I was considering was a very odd size for this area, so not just lying around. A more common size, might be a matter of just checking with the shop for a take off the right size. Also check Craigslist for used tires. If I did change size, and I may in the future, I'll probably find a used spare in decent shape. Unless of course I can show it off on a rear carrier, then I'd go new like you mentioned.
My wife says just roll with all five of the same tire because when I wear out the original four I'll be able to avoid buying the fifth tire the next time, assuming I go with the same tire.
Not a bad idea in theory, but what if you do want a different tire? Also consider tires age even if not used, so one tire will be older than the others and perhaps wear differently or be weaker. I'd do it in a heartbeat if TPMS wasn't a consideration and needing an extra matching wheel for a 5 tire rotation. Hence why I didn't buy 5 myself.
