Clean Install?

I have recently switched from PC to MAC (actually waiting for my Mac to arrive any day now!). I have one question though...is it necessary to reformat a Mac HD like I would on my PC? I find that I have to do a fresh install of Windows about every 6 months as my PC gets bogged down with stuff. Is this the same on a Mac or does a Mac stay "cleaner" as you use it (if that makes sense..)?

Thanks!