The rule is if you are not going to use the computer for more than 16 hours, turn it off. It saves wear and tear on the power supply bearings and other moving parts.
Networks keep theirs on all the time, but they usually have a UPS to protect the system in case of a power failure.
Try to avoid turning the computer on and off more than 1 or 2 times a day. If you decide to keep it on, turn off the monitor. It uses a lot of power.
Also, if you have an energy saver monitor, you can set your screen saver to shut off your monitor after a certain time of inactivity.

---