I don't scan my computer anymore for viruses, malware, etc. and it's been like that for over two years now. If you're careful enough with what you're doing while using your computer than you should be safe. I've never had any problems with my OS during my last two upgrades to OS X 10.7 Lion and 10.8 ML. It'll likely continue until the next version which should be 10.9 or something.
Just enjoy your computer and that's it for me.