The problem, or the solution depending on your point of view, is that Rails is not backward compatible between minor releases. Each minor version is work to upgrade to it. My recommendation has always been "start with the latest version of Rails even if it's RC status" since upgrading will be a pain. Hopefully the RC will be complete before you need to launch.

Then you have to make the decision for each app - "Do I upgrade or stay on the latest version"? I personally prefer that tradeoff - Rails can cleanly add new features even it breaks the past. Then I decide if I need those features more than I need the pain of upgrading.