I have been using Mozilla and Safari exclusively for 3 years now, and as long as Java and Flash are installed properly, I have yet to see a non-Xrated site display improperly. Xrated sites, well, that's another story , but very few of them display improperly.
In fact, the only time I have to use IE to visit a site is when checking for Windows updates at the office, since MS doesn't allow you to do updates from Mozilla.