Rule of thumb: forget about the version 4 browsers, unless you want to provide something like limited font tags. Even then there are inheritance problems, but the use of these browsers is so limited now (except, as noted by others, in some corporate in-house environments... Optus, for example) that you are wasting your time putting any effort into styling for them. Better to use an @import to rule them out from the start. (They will then get nice, clear HTML pages with no styling.)
There is no NN5.
IE 5 Mac was, for some time, the gold standard browser, but it is now orphaned. In the meantime, pretty much all Mac users on System 8 and 9 are probably using it, if not NN4. They could use NN7 if they were motivated and knowledgeable enough to get hold of it.
On OS X, many earlier users are still using IE5, because Safari appeared later in the piece. And there are still some sites (some areas of banking sites in particular, where Live Connect (pardon me if I am wrong) type calculation functions still don't seem to work properly in Safari) where IE5 is needed.
So you need to know the limitations and bugs in IE5 Mac (along with those in all the PC versions).
You can find some info here: http://www.macedition.com/cb/resources/macbrowsercsssupport.html and here: http://www.macedition.com/cb/ie5macbugs/index.html
IE4, NN4, NN5, IE5Mac - I've noticed these are mentioned a lot
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