[Andrew]

I like to design for both options and give my users the choice. Many people like a fixed width style, but many people also like a fluid width style. It also depends on the type of forum you're making. A forum that's going to frequently have large images within posts is usually better off with a fluid width style, as you'll frequently run into issues with the images messing up the layout of a fixed with style once they get beyond a certain size. I think this is one of those things that has no better or correct answer - It all depends on your target community and purpose really.