Essentially, whatever answer you pick is going to be the right one. Just keep in mind the costs involved. I had a PC that ran 24/7/365 for four years until every fan and the CMOS died on me. Now I have to retrofit it with new, non-proprietary, fans.
Barring any overclocking, mechanical things like the fans and hard drive will fail long before the PC itself does. Which is what happened to me, my fans ran for so long that when I finally powered them down for the first time, and let them cool off, they siezed.
Does that mean I won't run a PC 24/7 anymore. No, it just means I'll be more careful in the selection of my components.
It's a balance you have to pick between availability, longevity and operating costs.
My basic rule of thumb is, if the PC isn't going to be doing or serving anything in the next hour, then shut it off. This nets me two PC's that are on 24/7 and the rest get shut off while I'm at work or asleep (unless I wake up in an hour

).