Not sure why anyone would use such old versions of IE & Netscape. IE in my opinion is a rubbish piece of software for browser compatibility and if Microsoft are such experts then why do they make a browser that still isn't as good as Mozilla Firefox? Anyway, I personally only check my Web templates in IE7, Mozilla Firefox & the latest version of Opera. I would think that people are still using IE6 (and maybe even IE5) but only for those casual surfers who do not have ground-up experience in the Internet, so looking for site compatibility in such older browsers is clearly not necessary. You can check
this source for usage shares of browsers and as per usual IE dominates that aspect. Typically, if you design in Mozilla Firefox it usually renders much better in IE.