As an ex-hardcore gamer and overclocker keeping their machine running in top shape, I was one of those that made extensive use of blackviper's site to turn off all the misc services, spent 4+ hrs once a week running a defragger, a/v scan, spybot scan, adaware scan, emptying all the temp folders, keeping track of startup items, blah, blah, blah.
I brought that mindset to OS X and can tell you now, it is not required. I run Onyx maybe once every couple of months or when I notice a little slow down.
OS X apps just do not load stuff into startup the way it seems every Windows app does. That's just something you really don't need to worry about.
After trying all the freebie disk inventory apps, I agree with cwa, WhatSize is worth it's price.