I'd guess it depends on the amp and circuit design, but from what I understand turning tubes on and off does put stress and wear on the tubes above and beyond leaving them on. Obviously if you leave them on for several days without listening that point is moot. The best advice I've read is; if you plan on listening on and off for a few days, leave them on 24/7. If you know you won't be listening over a period, turn them off. My 300B SET's sound best after about 2-3 hours of warmup time. I don't notice much of a difference after that. YMMV.
Marco
Marco