This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
If you are writing software that reads feeds then you can't ignore RSS 0.9X, 1.0 or 2.0, but if you are providing feeds then you could certainly choose to offer only Atom 1.0 format feeds -- all of the commonly used feed readers and parsing libraries support Atom 1.0 now.
Of course, if you have to support older feed readers and legacy RSS apps you might have to be more cautious. Along those lines, check out the Intelligence Community Metadata Working Group's recent recommendation regarding RSS vs Atom. They recommend transitioning from RSS to Atom by starting to make your RSS feeds more Atom compatible (they call it "Atomic RSS"):