I leave mine on 24/7. Quotes from Upgrading and Repairing PCs by Scott Mueller:
Frequently, powering a system on and off does cause deterioration and damage to the components. This seems logical, but the simple reason is not obvious to most people. Many believe that flipping system power on and off frequently is harmful because it electrically "shocks" the system. The real problem, however, is temperature or thermal shock. As the system warms up, the components expand; and as it cools off, the components contract,In addition, various materials in the system have different thermal expansion coefficients, which means that they expand and contract at different rates, Over time, thermal shock causes deterioration in many areas of a system.
For a pure system reliability viewpoint, it is desirable to insulate the system from thermal shock as much as possible. When a system is turned on, the components go from ambient (room) temperature to as high as 185 F within 30 minutes or less. When you turn the system off, the same thing happens in reverse, and the components cool back to ambient temperature in a short period of time.
Thermal expansion and contraction remain the single largest cause of component failure....
Now, I am not saying that you should leave all systems on 24 hours a day. A system powered on and left unattended can be a fire hazzard...
Based on these facts, my recommendations are that you power the system on at the beginning of the day and off at the end of the day. Do not power systems off at lunch, breaks, or any other short durations of time. If you are a home user, leave it on if you are going to be using it later in the day or if instant access is important.
No matter what, these are just guidelines; if it works better for you to leave your system on 24 hours a day, seven days a week, then make it so.
End of quotes..