Generally speaking, any circuit will last longer if it is not continually turned on and off. Switching the power decreases the life of electrical components due to the stresses of heating and cooling.
In addition, newer LaserWriter models include energy saving modes that allow the printer to remain on, and still use very little power.
In Apple's offices, most LaserWriter printers are powered on for months at time. Some have been on for over a year with no need for increased maintenance.