Use the logs
Of course it's good to support the browsers used by 99% of your users, but consider how much work it will require to support the last 1%. Also, use logs to find out which browsers are in use by users of *your* site, instead of guessing or deriving from broad statistics of browser market share.
I had a boss tell me to "support all browsers". Really? Even Netscape 4? "Yes."
I went to the Apache logs and enabled user-agent logging for a couple of days. Then I produced a report of user-agents and the number of distinct IP addresses of each one. There was actually one user still using Netscape 4.
I told my boss it would be less expensive to send me to that guy's office, upgrade him to Firefox, and train him to use it than to develop and test an alternative web presentation for Netscape 4 (meaning no CSS; back to nested tables and blank spacer gifs, for those of us who don't remember prehistory).
