If you are confident about being able to analyse the markup and CSS generated by Fireworks and replace any that could be improved, give it a try.
I think the code is incredibly ugly and obtuse. The style names are horrible and everything gets its own style, even places where you would apply a common style. If you have any idea of what you're doing with HTML and CSS at all, you're probably better off doing what you're doing. The code is fragile and is meant for mockups, not live sites.
But, hey. It's easy to hit the button to export a page and just see what it looks like!
I'm only having minor issues with Fireworks CS5's code... nothing like what some have described. If you use good visual design sense in PhotoShop thru Fireworks, the code will be close enough that only minor modifications will be needed. 99.99% of people couldn't care less if the Web page they are visiting has "pretty" code anyway. If the site's pages hold together in Adobe Browserlab's collection of browsers and pass the W3C Markup Validation Service scan, that's good enough for me.
I have built lots of computers and the whole "pretty" code thing reminds me of a computer wiring job. Some guys are really good at it, and yes it does look good IF you open the side of the case or have a transparent side panel. When I wire a computer, of course, I make sure that everything is connected properly. Beyond that I make sure that there are no globs of wiring tied together restricting airflow. That's it. The computer is a tool to enjoy and get work done on. Similar is true of a basic Website. Obsessing over code reminds me of an episode of "Monk" where he is trying to interview some crime suspects in a corporate lunch room. Monk was so distracted by the uneven levels of coffee in two carafe's sitting side by side that he poured the decaf into the regular to even things up. He totally distracted himself.
Coding to me is tedious like doing income taxes. Some people love it though. I remember a guy who hand coded a beautiful golf related website back in 96'. The site looked great until a new version of a popular browser came out. The site completely fell apart. What a nightmare. From about the same era, I remember a programmer who did an excellent job on the back end building a database driven Website. Some Web designers were just beginning to use Flash. I remember, this programmer liked Flash so much, he had little Flash flying saucers hovering about his personal corporate Website. It was just plain freaky.
I have built many websites the old school way using PhotoShop, ImageReady, Flash and Notepad. None of the sites I have ever built using Adobe tools have fallen apart in a browser like my old hand coding expert friend's site did. I haven't seen Web page code exported from Fireworks (CSS and images), and fine tuned with Dreamweaver fall apart in any browser or generate errors in the W3C Markup Validation Service scan.
Computers excel at crunching/cranking out code. They do this much quicker and with greater accuracy than people do.