Multi-categorization? (I.e. crossposting?)

Gnus lets you file email messages go into multiple groups -- e.g. if a
piece of mail is CC'ed to two lists, you can file it into archival
groups for both.

Has anyone experimented with filing messages into multiple classes
simultaneously?  What would be reasonable criteria?  Jason, do you
have a sense of how large the distance "typically" is between the
scores of "good" and "bad" matches?

Jeremy