Macs don't have the registries that Windows PCs do, so you don't get the same file clutter.
Running OnyX too often can slow things down a fair, as clearing the caches, etc, makes it take longer for things to launch and run until things have been run a fair bit.
Basically, in the early hours of the morning, OS X runs it's maintenance scripts automatically.
If you always allow your Mac to shut down or sleep during these hours, then using OnyX every 2-3 months is a good idea, otherwise, it is really not essential.
See
http://www.thexlab.com/faqs/maintscripts.html
There is really nothing to do in majority of cases but to sit back and enjoy using your Mac
*I never turn off my Macs unless a software update forces me to do so, or if I travel by plane, otherwise, I have the display set to sleep after 3hrs of no use and computer set to sleep 'never' unless I close the lid which I do on occasion.
Never had any problems with my Macs