There are 2 main groups to target. (There are plenty of others though)
Group #1 is browsers that use Webkit (Safari for example), Presto (Opera for example), KHTML (Konqueror for example) or Gecko (Firefox for example). These browsers should all get the same markup, CSS and Javascript code (as they're all in the same group of standard-compliant browsers). Only work around bugs in one of these if you absolutely have to and have the resources to do so. Instead, test in the latest stable versions of each (as you're developing so they can keep each other in check as to what the expected behavior is) and (after checking in the nightlies for the bugs) file bug reports. Again, avoid workarounds for a specific browser if you can. Instead, plan a cross-browser compatible solution from the beginning.
With Group #1, you don't have to worry about older versions much, if it all.
Group #2 is browsers that use Trident (IE for example). Target IE versions you care about and still only workaround the most severe bugs.
Also, don't deny browsers you don't officially support. Let them fend for themselves instead of blocking them (either intentionally or through crappy browser detection).
Also, remember that when looking at market share percentages, try to figure out the numbers they represent so you can see how many millions of potential visitors with that browser there are. 1% or 5% might not seem like a lot, but that could still mean millions.
Most of all, listen to the visitors. If you're getting multiple complaints about a certain browser, look into it if you can. Even if it's for a browser with low market share, if it's a trivial fix, you should just do it.
Ones that are definitely not dead are: IE6 (starting to push it), IE7, IE8, latest Opera 9.x, latest FF 3.x, latest Safari 3.x and others that have about the same capabilities. FF 2.x isn't dead either and is needed for Win9X users (if they don't want to use Opera)