I can safely say that it is an excellent idea to fully reboot your computer once a day. This means restart it completely, not just wake it up from sleep. Alternatively, log off, then back on (for faster yet let thorough results).
The reason is simple; Lots of third-party applications have memory leaks. When these programs stay resident, they slowly consume your virtual memory, making your system more and more sluggish. Eventually your page file would be expanded and you'd fault when you ran out of disk space. Anyway, I find that a reboot once a day for most 'crap' PCs is often quite desirable.
Also, if there is some problem due to a newly installed application, you may not know about it until a reboot. If that reboot (restart) is a week later, then you may have trouble remembering what you last installed or changed.
I can't help but plug my own software, which will help restrain unruly background processes to help ensure a responsive PC experience -- amongst other process related tasks.