It depends. I'm a firm proponent of the theory that says "If it ain't broke, don't fix it!" In other words, if your not having problems, don't reboot the server just for the heck of it.
You will probably here from many administrators that will tell you that they reboot their servers on a regular basis as a preventative maintenance measure.
If your servers are running smoothly, you aren't seeing error messages popping up in your system and application logs, and you aren't seeing any noticable process bottlenecks creeping in, then you really don't need to reboot on a regular basis.
You will run into problems where your only recourse may be to reboot. But in my experience this has been the exception rather than he norm. Your mileage may vary.