I was having the same center alignment problems and came up with a hacky workaround - blog post

Note: Tested in Fireworks CS5 on Mac, but should work approximately the same in other versions or Windows.

Open /Applications/Adobe Fireworks CS5/Configuration/HTML Code/Dreamweaver/SLICES.XTT and locate the following line: (line 662)

WRITE_HTML("<!--Fireworks CS5 Dreamweaver CS5 target. Created ", d, "-->

");

Now add the following right below it:

WRITE_HTML("<style>body {text-align:center;}</style>

");

and BAM! You’ve got centered exports.