I do different things for different PCs. My main gaming PC is only turned on when I'm using it, or when I have it processing something (transcoding, etc). Even though I made an effort to use somewhat efficient parts and settings, a high-end PC with a decent GPU, OC'd proc, big PSU, etc is going to eat power. So it's probably on an average of 6-8 hours per week (not much time for gaming these days) all in one rush then gets powered off.
My laptop on the other hand is my main email, internet, chat pc - it is powered on 24/7 unless I'm leaving the house for more than one overnight. And I have hibernate/sleep disabled on it, so it's on on. It even only gets restarted when I absolutely have to (usually because of MS updates).
My media center I usually set to "sleep" - that way it comes back fast (even though it would anyhow with an SSD) and isn't using much power generally.