generally Mac OS X takes care of its self as its Unix core runs maintenance scripts and defags the disk. great isnt it? you can use onyx to do manual cleaning tasks every once in a while if you want.
repairing permissions every now and again wouldnt hurt but its not essential. it will fix boot problems if a system file has the wrong permission. veryfying your disk for errors is a good idea. if an error is found repair the disk by booting into the install DVD and using disk utility.
im a very strong believer in not using AV for my mac. there is no need for it as there is are Mac OS X viruses in the wild. even if lethal virus did appear i would not go on any of the sites where the virus was found. macrumors and other blogs would be bombarded with news showing the precautionary steps to avoid infection and how to get rid of it.
generally i am my own virus scanner. i would never give my admin password to anything if i was uncertain what it was doing. a virus or trojan horse can only do damage to Unix systems if the admin password is given unlike windows.
if you really want to use an AV i would go for an open source AV like clamxav as the commerical AV like norton and avast are a waste of money and usually bad for your system.
