On my old Dell of almost 6 years (and my old Gateway before it), I have the habit of shutting it down when I go to bed. Once I start it up in the morning I leave it on all day. Sometimes it stays on for more than a day at a time, like if I'm doing a scheduled weekly overnight virus scan that takes a long time.
I think it is good to "exercise" the computer this way. Here is why: As you know when metal heats up it expands and contracts when it cools. Particularly the hard drive(s) which get very hot.
I had a buddy of mine 10 years ago after we bought identical Gateway desktops at the same time. I turned mine off every night while he left his on all the time. Well that Summer after buying it he went on a family vacation and decided to turn it off while they were gone, in case of storm related power failures and stuff. When he got back after a week and turned it on it wouldn't boot! The hard drives were so hot running all the time that being off a week caused the metal parts inside to over-contract and warp!
Plus, from my own experience before that, I had a computer that I did run 24 hours a day to host a dial-up BBS before the Internet days. I remember going through several hard drives along the way.
Maybe there were just crappier unreliable components back then though. Now I guess with modern computers having powersave modes, that may help some.
But I'll be getting my new gaming PC tomorrow and plan to do as I have always done.