As others have said, being on all the time ages the tubes prematurely, but on/off cycles are probably harder on them than just being left on.
I compromise - I turn my system (amp, pre and DAC - all tubed) off every night during the week, but I leave them on all weekend. I figure this works out best, since I often don't listen for a couple of days during the week. This regime minimizes the on/off cycles and the amps are always up to speed when I want them on the weekends. While the power tubes only take about half an hour to warm up most of the way, the amps really only sound their best once they'be been on for an hour and a half or so.
I compromise - I turn my system (amp, pre and DAC - all tubed) off every night during the week, but I leave them on all weekend. I figure this works out best, since I often don't listen for a couple of days during the week. This regime minimizes the on/off cycles and the amps are always up to speed when I want them on the weekends. While the power tubes only take about half an hour to warm up most of the way, the amps really only sound their best once they'be been on for an hour and a half or so.