Can't believe almost every one of these recommendations is saying to NOT restart your computer on even a semi-regular basis. I have a rMBP which I restart at least once a week. I also have a Early 2008 iMac that acts as a server that I restart at least twice a month. I do this b/c it allows the machine to purge unused memory and temp data that builds up over time.
I work in IT for a large, Mac-based school district. I cannot tell you the number of times that a simple restart fixes simple issues and how much better my staff's computers run if they restart once a week. On average, especially if your machine gets used quite a bit; I would recommend restarting your Mac (or any computer for that matter) at least once a week. A lot of it depends on how you use your Mac.
In the end, if you're Mac isn't a server, then restart it every once in awhile. "Always-on" is meant to be for Servers, and even then, servers have to be restarted from time to time.