Possibilities include:

  .../rss , .../rss.xml , .../.rss , .../rss_full.xml , .../feed , .../rss-feed , .../feed/all/ , .../MySection.xml , .../MySection.atom , feedserver.example.com/section/index ...

I've yet to see a Tumblr site that didn't have working autodiscovery. But maybe I just haven't seen a custom-enough theme yet.