I'm starting with an existing .doc MS. Kindle warns to not use hard para breaks, i.e., don't hit return, but I can't get rid of any of them. Driving me nuts.
For ebooks, the general rule is, don't use two consecutive returns (as @Fortier mentioned in the comments). This is because the ebook readers sometimes do weird things with it.
However, this is just a suggestion, and almost every Ebook vendor will allow it. Kindle only moans if you enter KDP.
My advice is, write the book, and worry about formatting later. You can easily remove double returns using regexes, which works in Word and every other processor. Google can tell you how.
PS: I'm pretty sure I've answered this question earlier, but I can't find it now. Anyone else remember where it is?