I was on my highschools tech support staff and we left all those machines running 24/7. I went back to visit a few months ago and there are computers in that building that are nearly as old as I am and still running the original processors. (They're slow as all get out, but still running).
I generally leave mine running 24/7 for bittorrent or Ares/emule and I haven't noticed anything. I do shut it down atleast once a week because there are always random things that happen that are fixed by restarting.
I wouldn't be worried about the CPU when leaving the computer running all the time, but rather the amount of spyware that can accumulate. I run spybot atleast once a week if not twice.