Swarming advice needed

If my guess is correct (swarming doesn't seem to be
documented) it tries to download chunks of a file from
multiple sources (if available).
This does not seem to work very well for me. I suspect it
may be related to my using a modem. I know from Gnucleus
that such multiple source downloading is NOT recommended
for modem users. Yet GTK-Gnutella's tooltip seems to
strongly recommend using swarming.
Any advice for a modem user? Should I keep swarming on or
should I turn it off?
Most of the files only ever have 1 source anyways.