I just do it in Word. I convert the Word document to HTML with my macro. I do not use the save as (filtered) HTML in Word. I import that in Sigil and I continue there.
I have also done quite some tests with DocToHTML and that produces rather clean code and you can immediately convert it to XHTML on the go. It can also create a stylesheet.
Others use OpenOffice with the Writer2Epub add-on.