extensionless URLs for feeds?

I'm setting up a new site and thinking about URLs. I've pretty
much decided to use extensionless URLs for regular web pages[*],
but I'm not sure what to do for feeds. Is there any current
recommendation or best practice for feed URLs? I've seen all of
the following extensions used:

.atom
.feed
.rdf
.rss
.rss2
.xml

I've also seen extensionless feed URLs as well as feed URLs that
are directories (URLs ending with a slash).

What are your thoughts about this? Any recommendations?

Thank you,
Nancy
[*] I've written about the .htaccess settings I use for my
extensionless web-page URLs here:

htaccess excerpts and notes
<http://deflexion.com/2008/03/htaccess-excerpts-and-notes>