Unless the spec is still in flux and the unprefixed syntax changes, then you're SOL.

Still better to have the unprefixed version in your code than not to, I'd say. I really doubt the spec would change so that old code was still valid, but had a different meaning; most likely, the worst that would happen is old code would become invalid (and CSS's error handling makes invalid code not a big deal).

And put the unprefixed version below the prefixed properties, so it has priority in browsers that support both.