If you use Notepad, line breaks will be converted to <br></br>
If you add line breaks in your WYSIWYG they will be <p></p>, not <br></br>, which will cause inconsistency in line spacing.
This little piece of information, the distinction between <p> and <br>, also came very, very useful, since I by chance took one more look at the
parameters of TinyMCE plugin, and found an option there for setting new lines as either P elements or Br elements...
...I set it to Br elements and now I can control line spacing the way I want! For some reason, this has stopped working...