I learned recently that the mark-up I had been doing to get rid of line breaks between line groups in poems wasn't quite right, so I just wanted to post the correct mark-up as a reference point for myself (and others who might make the mistake, too).

Instead of using:

lg type="stanza" rend="margin-top: 0; margin-bottom: 0;"

On the top line group, I would want to use:

lg type="stanza" rend="margin-bottom: 0;"

... and on the bottom line group,

lg type="stanza" rend="margin-top: 0;"

And this mark-up would get rid of the space between these two line groups!