I came across the advice of a Mac consultant who wrote that you should restart your Mac once or twice daily to prevent your Mac from slowing down. This seems like a rather dubious way to fix a problem. Your Mac should not be slowing down as you use it. There are a variety of reasons why a Mac might slow down over time, such a memory leak in a poorly written application. However, restarting your Mac is not a solution, just a band-aid.
OS X has Unix underpinnings. Unix is designed for reliability (i.e. high uptime). Some folks have kept their Macs powered on for over a year. This isn’t necessarily recommended, since you’ll need to restart your Mac for occasional software updates. But don’t feel compelled to restart your Mac daily. You may want to put it to sleep to reduce your electric bill.