pawright
are you talking about a 2nd spare wheel, or THE spare wheel.
If its for a 2nd spare wheel, i'll be putting mine on the roof racks - as it will only be up there if i'm doing trips where i want a 2nd spare. Most of the time it will stay at home. Not too worried about a bit extra fuel consumtion, as i'll already be loaded and towing a campertrailer - so fuel ecconomy will be crap anyway. I might even be able to put the 2nd spare on the campertrailer ... still working on that.
If your talking about the original spare (located under the tray), i dont see any need to locate it elsewhere, even if your stuck in mud. If your needing to change a tyre in the mud, you will struggle to safely lift the vehicle anyhow - i'd extract the car from the mud before worrying about changing the tyre.