I've had no problems posting from either Open Office or Word (depending on which computer I write on - Linux or Wondoze). The formatting always stays the same from source to destination. Where a person runs into trouble is MODIFYING text already posted with edits inside TNBWs edit panel. Formatting can, and most like will, go wrong doing that.
If I decide to alter anything at all in a chapter post, I highlight ALL in the source document, hit COPY, then move to TNBW and highlight ALL inside the edit panel. Then, choose PASTE. This will completely replace everything you've posted previously with the new version, and thus preserve everything as you want it. I rely somewhat heavily on italics and, as you say, they are not kept using Notepad. This process also preserves the complete copy of your source document should you ever need to recover it from TNBW for whatever reason.
Bill