The 2 PC's I use as DVR's are always on, my desktop computers are generally on all day and I shut them down when I go to bed unless it is doing something, like running virus malware scan, cleaning up free space etc. The netbook powered up only when it is being used.
However when there is a thunderstorm in the area all computers and valuable electronics are powered off and disconnected from the AC at the wall. If you have an overhead cable line to your house I would remove the cable from the modem, Cable box & TV as well. I have seen way too much stuff zapped by a passing thunderstorms.