There's no need to shut it off, but it is a good idea to restart the system to force it to repost, retest ram, check plug and play, make sure hdd is still detecting, clear hung sessions in RAM, etc. every so often (..maybe at least once a week).
Wether you decide to power down or not every day, just as everyone has mentioned above, a good surge protector is a must. Make sure all items (monitor, PC, modem phone cords, any AC/DC adaptors that power PC devices) are plugged into the surge protector.