We never see dry rot on indoor stored vehicles and spare tires here in NorCal. Never here at the Oasis.Do you plan to replace the unused spare when it dry rots? To me, that's just a waste of $. I'll rotate all 5 tires on my LC and replace all 5 when the time comes. With TPMS on the spare, I'd know if it's not holding air pressure. Plus, since I'll be doing the rotation myself, I'll inspect the tire that becomes the new "spare" when I rotate. Nothing wrong with keeping it "perfect", but you should inspect and replace the spare when the rubber begins to deteriorate (typically within 5-6 years). I'd wager most people never replace the spare.
I wholesale out entire sets of tires before they wear out so the entire subject in kind of irrelevant in my case. Trying to give the OP some guidance that would possibly save her some money and more fun.
Depending on her use profile this is a subject the could go on and on.