I'm with Steve: HTTP already has plenty of ways to handle caching, don't invent something new for RSS/Atom aggregators. If they just follow Mark's rules (handy tests and instructions), life will be fine.
I worked hard to help HTTP caching on my blog. It's complicated, particularly with the pastiche of dynamic content I have. Used to be 40% of my weblog requests were answered with a bandwidth-saving 304. When I added my linkblog it went down to 25%, probably both because the HTML view changes more often and because I removed ETags support.
Most aggregators do fine. Radio Userland is having trouble since I turned off ETags. And NetNewsWire has a surprisingly low number of 304s, although a quick inspection doesn't show anything obvious.