I leave my mini all the time, and only re-boot when I get updates from Apple.
I'm slight peeved that I have to reboot after the updates, as I don't have to on Linux, but it isn't that often. The only things I would check if it's slowing down are the number of items on your desktop, and what you have starting when you boot. I.e. how many apps are running at the same time. Cheers, -Alastair
