Originally Posted by jackie_w
It doesn't really matter whether you add blank lines or not. The main thing to give yourself an easy life when using Word to create HTML (filtered) to import to calibre is that you use the Word built-in styles 'Heading 1', and/or 'Heading 2', and/or ... 'Heading 6' to style your chapter headings.
jackie_w is absolutely right. When using Word, if you just do those two things: 1) use the built in heading styles , and 2) save as filtered html, Calibre will be very happy and conversions will go well.