I agree with Shaman. I partition my drives like so:
1) MacOS (whatever is installed with an install of your system software)
That way, it is easy to back up just what you need and keeps things logically organized.
But to get back to your question, I probably rebuild my desktops twice a month, and defragment just as often. I like to defragment simply because I like watching Speed Disk's graphic representation of the drive being defragmented.
It also gives me a warm, fuzzy feeling to know that everything is in tip-top shape.
I also format the drives whenever it makes sense, like you said, perhaps when too much junk has been installed, or if I haven't formatted in a long time. I think it gets rid of whatever junk has built up, and again, gives me that warm, fuzzy feeling that comes from knowing everything is running as smoothly as possible.
As far as anti-virus software goes, I guess I don't really use any. Probably not smart, but I haven't had any problems with viruses since CDEF or WDEF (can't remember for sure) infected all the floppies at our high school. That doesn't mean I won't in the future, but I'm keeping my fingers crossed and taking some precautions to avoid viruses in the first place.
So, in short, it isn't going to hurt anything if you rebuild your desktop or defragment your drive quite frequently. Do it when you feel it needs it, or when you suspect that something isn't quite right and could be fixed by the above processes.
And, if you have any sources for free/shareware defragmenters for the Mac, please let me know. I always like to try out new things like that, to see how they compare to the ol' standby (Norton).