This item is from the PC Hardware FAQ, by Willie Lim and Ralph Valentino with numerous contributions by others. (v1.25).
Turning a device on/off causes thermal stress. Leaving it on causes wear and tear (even on non moving parts). The only thing you shouldn't do is quickly power cycle the computer. If you turn it on/off, leave it that way for at least a few seconds. Other than that, it's up to you.
 
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