In theory, you should reboot regularly just to clean up the memory, close down old programs completely and basically gives you a fresh start, but with the newer operating systems, many of the lockups and instabilities have been eliminated. In reality, reboot when you add, update or patch but otherwise, with a newer machine (Windows 7), you really don't need to reboot to improve the computer's operation.
Now there are some benefits to rebooting, so doing it occasionally or at night, will not hurt it at all. Shutting down will save a few dollars per month in energy costs. Putting your computer to 'sleep' or 'hibernate' uses very little energy and allows a quicker start up.
So, do you NEED to shut down your computer on a regular basis... no. It is your choice for the most part. Isn't that nice?