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SciFiWriter - I do all content editing in Word, in part because I sometimes need to produce a post-facto PDF or XPS version and Word's the best place to do that. But mainly because I'm more comfortable changing sentence structures, making grammar and punctuation corrections, in Word than in an EPUB code editors. My 'books' tend to have a lot of end notes, long reference lists, and indexes, which generate lots of markup, which in turn gets in the way of editing the actual content.
My layout needs are fairly straightforward, so I would rarely need, be tempted, to make markup changes in an EPUB editor (calibre or Sigil, I use both). I find their spell checkers useful, and the facility where I can get a list of hyphenated words is also useful - but I make resultant corrections in Word. I also use them to 'debug' the odd conversion anomaly - which are invariably my fault, which I fix in the original DOCX.
I convert DOCX rather than Filtered HTML with calibre. My experience is that the output EPUB is 'a little more faithful' to the original - assuming you use Word's styling features. Kovid Goyal (calibre author) ranks DOCX just above HTML in this list
Best Source Formats to Convert
Another thing you might want to consider is Toxaris'
e-Book Tools - a Word add-in. Aficionados claim that its DOCX->EPUB conversion produces cleaner HTML than calibre, I don't use it because I've 'subconsciously' configured my Word Styles to suit calibre conversion. However its Dialogue Checker and Search and Replace are invaluable, and it has a more than handy EPUB to DOCX converter.
If I were to start editing the EPUB it would become the master, and I would probably archive the .DOC(X) and remove it from calibre. If push comes to shove I could use Toxaris' EPUB to DOCX converter if/when substantive revisions result in me wanting to use Word on an existing work.
Another reason I like to keep the DOCX as the 'master' is that they get content indexed for Windows Search, whereas EPUBs do not.
BR