In a network with NMEA equipment from different manufacturers in which
some equipment has a real, separate NMEA (-) lead and other equipment
that takes the cheap route and uses DC ground for the signal return, is
it best to just leave the NMEA (-) leads floating, or run them to DC
ground?

There are no doubt some combinations where leaving the dedicated NMEA
(-) signal return floating may be best, but In my experience, tying it to DC
ground worked best.
I can imagine that with a "noisy" DC ground system, or one in which
other data systems dump data return on their DC neg power connection some
data noise is inevitable. (Why do they insist on doing things like this,
anyhow?)
In simpler terms, try it. It USUALLY works without letting the smoke
out.
Old Chief Lynn W7LTQ/ PG-13 20604