I would advise against that. You should code by the standards and not cater to specifc browsers. It's mostly the IE developers that got it wrong, so after you coded your stylesheet to the standards, then proceed to code to the browsers that are closest to supporting standards (Safari 3, Opera 9, Firefox 2) and then the rest.
But yeah, the exception would probably be to IE where most nowadays would use conditional comments and feed something like an HTC to fix PNG's, or use the filter property so the CSS validates (If you know what you're doing you don't need everything perfectly valid).
You should try to avoid hacks (feeding different values with proprietary selectors, etc), but then now that I think about it it's alright because that's how everyone starts out (using hacks).
Just try to minimize the number of hacks you use (shouldn't need to use a box model hack -- just add an extra wrapper... shouldn't need to use different widths/margins - floated elements with horizontal margins get fvcked in IE6 because they get doubled.. so you feed display:inline; to get rid of that.. for pixel perfect list items you need to float the LIs because IE has inconsistent spacing EVEN with a universal reset and that can only be solved by white-space:no-wrap; or floating the LIs.