Mine gets switched off too although the amps previous owner told me he left it on standby. If I can't get it sounding right it may never be switched on again
Apart from any warm up time, I believe electrical devices are most at risk of an internal failure at the point of powering up which in theory should mean leaving them permanently on might help reduce the risk but I'm too much of a worrier and wouldn't sleep if I'd left a 15yr old amp switched on while my kids are upstairs.