Ever since Mac OS X has been released, nearly all the macs that are supported are bulletproof under it. You can leave it on for days, weeks or months at a time, and it won't slow down or crash like other operating systems (ie OS 9, Windows).
first off get a UPS a.k.a. battery backup which will save you from power outage. having a battery backup and only occasionally powering down your computer should make your computer last longer. beside that, any computer designer, designs their computer to run 24/7, you won't hurt anything.