Does running MP 24/7 slow it down? Not recommended?

I think this is a general Mac question and not a Yosemite one, but I have a question. I'm in the habit of leaving my computer on 24/7. After awhile the Mac Pro (converted 2.66 to 3.46ghz, 32 gb ram) gets sluggish. I also leave a bunch of apps open. Lately I've been looking at the Activity Monitor to see whats going on. CPU time for Finder and Window Server are about 21:07:53:XX. Should I shut down every night, day or so? Does running it 24/7 run up CPU stress?