I figured you were trying to save money. For my pvr that actually does the recording, it's on 24/7/365. I think of it like an extra fridge.
In the evenings I have it archive (transcode) shows I want to keep. I have a nightly backup to my WHS. I also have it set to do any updates. So for me while I'm sleeping is usually when the computer is doing most of it's work in my day.