Actually leaving a computer on all the time is better than turning it off and on a lot due to chip creep, and with modern computers now having energy saver technology it actually doesn't take so much electric if you do. What happens is the PCB's tend to expand and contract, causing things to not match up so well after a time. Similar to the reason they always put seams in concrete sidewalks to allow for changes due to temperature. To be honest though I don't think chip creep is as bad as it was in the 80's. I still see a DIMM chip creep out now and then on computers I work on. (That's your memory chips.) Monitors definitely don't leave on as they can get burn in. Other than that it probably makes little difference if you're keeping up with technology because you won't have the computer long enough to worry about chip creep.