From http://www.newsreaders.com/guide/netiquette.html:

/Don't excessively cross-post/

* Cross-posting refers to posting a message to more than one group
at a time, in the same post.
* If a message truly belongs in multiple groups, by all means
cross-post. In fact it is better to cross-post than to post the same
message separately to different groups.
* Be careful when replying to a post. The initial post may have
been inappropriately cross-posted; if you do not edit the list of
newsgroups, your reply will also be inappropriately cross-posted.
* Consider setting "followups" to one group. Your post may be
relevant to 3 groups, but you can set followups to one group and then
the discussion will continue there. When using the followup header, it
is nice to put in the message somewhere "Followups set."

And from http://www.use-net.ch/netiquette_engl.html:

/Avoid posting to multiple newsgroups/
Few things annoy Usenet readers as much as multiple copies of a posting
appearing in multiple newsgroups.

NYCXYZ, please stop cross-posting or at least set the Followup-to for
one group. Thanks.

The followups to this message are set to rec.bicycles.misc, a group that
I rarely visit but seems most appropriate for this discussion.