I read somewhere that heat cycles will eventually make your tube fail, along with friction between tube/tyre caused by acceleration (back wheel), & also braking/cornering etc. I also notice that the metal parts of the valve stem, particularly nut & washer, go rusty quite quickly, so the part you can't see inside the rim, will probably be in even worse shape. As tubes are not much money, I reckon changing every second tyre change probably gives good peace of mind. I'm no mechanic, & I don't ride off road.....