I'm not sure what this is referring to? Do you mean when you hit ENTER?
Like this? If you hit ENTER, it starts a new Paragraph <p>.
This is a new <p>aragraph. But, If you hit SHIFT-ENTER, it will just break the line,
like this. I just inserted a <br />.
In general you should use <p>. A feature request that's going to be implemented sooner or later is customizable stylesheets, so you could change paragraph spacing if you don't like it. I would suggest using that over line breaks.