This is an interesting question that I have pondered over for a long time, can one stylesheet really fix all cross browser compatibility issues?
Recently I worked collaboratively on a web application that needed to be compatible with IE6 users, as well as all the others and although I was tempted to use the old IF IE fix, I decided to stick with one stylesheet. Using various tips and tricks I successfully fixed all IE cross browser compatibility issues thus making the web app appear the same across all the popular browsers.
I would say that if you have a more complex layout and you want everything to be pixel perfect then it may be necessary to use the IF IE fix.
I will be writing a list of popular IE browser issues and their fixes, so check back again soon.