You'll find this thread to be of interest. IMO, the conclusion expressed by Cathode in his post dated 2-8-11 is the best possible answer to this perennial question:
-- Al
Depending on what kind of gear you have, how it's designed, and how you use it, turning it on/off vs leaving it on is truly component specific; making a blanket recommendation is not possible.... FWIW - I work as a reliability engineer in the semiconductor industry.Regards,
-- Al