If you notice that the lug nut mounting surfaces are getting rusty, scrape and wire brush them. my old Jeep Grand Wagoneer had teh spare mounted under the back, so i threw the spare on after a flat, and about 5 miles later, at 60mph it didn't take the turn that the rest of us did! the holes were so rusted that the lug nuts didn't seat, so the came loose quickly.
(Big jeeps are strong! didn't even feel it, my wife just thought we struck a pothole, but it sure takes a long time to stop!!)
i just put an aluminum rim under my wifes ex for just this reason, and plan to do teh same with mine when i find a matching spare
Scott