I don't really think there's a definitive answer to that question. I'd say that it largely depends on the site you're building and it's demographic. Code Scene, for example, is targeted primarily at web developers so I don't care if it doesn't work in Netscape 6. If you're a developer and your browser of choice is Netscape 6, then you have more problems than Code Scene can solve. On the other hand, if my target audience involved public schools and libraries which may have outdated machines I'd want to make sure that the site was accessible in older browsers.

"Accessible" should be the key word here. You'll go broke and/or crazy trying to get your site to look pixel-perfect in every browser. You should concentrate on making it perfect in the ones that count and making sure that at least the content is accessible (although not 100% pretty) in the others. You can do this with a wide variety of tricks such as using @import to include your CSS files. Modern browsers will render them. Netscape 4 will ignore it completely and render an accessible non-styled text version of the site.

Again, while the answer to the "What browsers should I test in?" question is different for each client, there is no question that some browsers are more important than others so this is my opinion on which should be prioritized when building a new site.

I've based this on browser data from TheCounter, W3Counter, Browser News, Market Share, and the analytics reports of a few sites I've built. If you're re-building an existing site, the analytics reports are a great way of seeing what browsers people are using.

Without further adieu, here are my recommendations for which browsers to prioritize.

Priority 1 - Your site MUST work flawlessly in these browsers since they account for the majority of Internet traffic.
Internet Explorer 6
Internet Explorer 7
Mozilla Firefox 2.0
Mozilla Firefox 1.x

Priority 2 – These browsers are somewhat older and/or less popular but your site should look good and work properly in them. (If you expect large amounts of traffic from Mac or Linux users then you may want to move Safari and Konquerer to Priority 1)
Internet Explorer 5.5
Netscape 7+
Safari 1.2+
Camino 1+
Opera 9+
Mozilla 1.x
Konquerer 3.4+

Priority 3 – These browsers are outdated and not widely used at all. Steps should be taken to make sure the site is accessible and/or downgrades to a text-only version in these browsers.
Netscape 6
Mac Internet Explorer 5.2
OmniWeb
Netscape 4.x
Internet Explorer 4.x
Lynx

Again, these are just general recommendations. Always keep your audience in mind when making the final decision. If you have agree/disagree, leave a comment below.