On, always. I move in/out every 3 months for school (co-op), and my linux desktop (C2D, etc) basically is settled in, turned on, and turned off 3 months later. Nearly the same with my laptop, its either running off AC in its dock, or I pop in the battery and take it around, returning it to its dock. Only time its off when its traveling really. Haven't noticed any ill-effects; upgraded/cleaned out cooling on the laptop, and the desktop is WC'd.