You should be OK. Keep it tight so there's no vibration or wobbling. Make sure that rubber bumper is tight against the wheel.
If the wheel can move, it will start "tin canning" on the tailgate - you know how a tin can is strong when it's straight, but if it gets even a tiny crease, it bends really easily? You're avoiding that.
A heavy tire is a stress on hinges, but thats' not horrible, especially if you don't open the tailgate a lot.