Yeah, I used to do the very same thing - leave them on 24/7, until I discovered how little power S3 sleep uses (its basically the same amount of power as shutting down), how quickly it resumes, and how much it cost me to actually run them 24/7 (hundreds a year). Now the only system I leave on is my server which pulls only 50W, all the other systems are set to go to sleep after 15 mins of idle, if I don't do it myself.
I'd highly suggest sleep over hibernate - hibernate is a really ugly solution that belongs only on a laptop you wont be using or charging for days (a notebook will last quite a few days in S3). It shuts down completely, but doesnt save much more power. It gets back to the desktop much slower, and even then it has quite a bit more loading to do, because it doesnt 100% return memory to its prior state - all the file caches get flushed, so you'll notice that theres quite a bit of slowness and grinding after a hibernate because of that. Sleep is a few seconds on or off, no grinding at all. It also helps if you live with others and have separate accounts - sleep can knock it back to the login screen on resume so other users dont have to log you out (or screw around on your account).