You should reboot only as needed.
However, some application may not release resources (PTEs, nonpaged or paged pool, working set or private set memory, etc...,etc...)This is known as leaking....
Tivoli and Domino server are two apps I know of that will constantly leak working or private set memory. When this memory usage gets too high, your server will slow and may dump. (Tivoli's documentation used to actually say that this is normal. I don't know if that is still the case)
I encourage admins to use perfmon every so often to see if any apps are leaking. Then you can be on top of problems that may occur.
Other than that, no reboots without a cause.
Microsoft Certified Nut.