Tubes if left running are still running and putting hours on them, plus generating heat. Thing is, if a tube's life is around 10,000hours for example, then theoretically you've got a year to keep that running. I'd worry more about the heat since it'll stress not just the tube, but also everything else inside the chassis. Living in a cold climate won't help much if your house conforms to building codes (meaning it's insulated enough inside so the climate outside doesn't matter that much).
In any case, what's convenient about leaving an amp running 24/7/365.25? I know how McDonald's, WalMart, and Detroit have conspired to make people move a lot less, but where exactly are you putting that thing to make that switch hard to reach? On my end though I not only switch off the amp, but I use a power strip with a switch on each outlet. I use my toe to flick the switch on my automatic voltage regulator, then my toe again for the appropriate button on my power strip, then finally my right index for the amp switch. Probably five seconds from the AVR to the amp, then I take a shower before listening (solid state but I still have this habit from when I warm up the tubes).
