One of the great things about RSS is that it's being used all over the world in countless ways. Millions of sites publish RSS feeds and hundreds of products consume those same feeds. The downside of this popularity, however, is that with that many implementations, there's bound to be some variance in how and when sites implement the specification. This can make it tricky for publishers and consumers to figure out how to implement some of the less-frequently-used features of the specification.
For the past few years, the RSS Advisory Board and the many hard-working and dedicated people on the RSS-public mailing list have been working hard to document how popular readers and services have implemented the specification, with the goal of helping publishers and consumers understand how best to get what they want.
The board recently published another draft (version 1.20) of the RSS 2.0 Best Practices Profile, which includes information on how various consumers use the <ttl> element, as well how to implement some commonly-used RSS extensions (like the Dublin Core and Slash extensions).
The profile is still evolving, so, if you're interested in getting involved, the best place to start is on the RSS-public mailing list. If you're implementing a new RSS publishing or consuming application, a quick glance at the profile would definitely help.