You should be looking at enhancing things with css3 so that good browsers get the best experience but older browsers get the standard experience.
Don't use any css3 properties that would break a layout if they were missing in certain browsers. Things like border-radius, text-shadow, box-shadow are just icing on the cake and do not break a layout if the browser doesn't understand them so you can use them safely.
For the more advanced, newer and experimental CSS3 properties I would wait until most of the browsers support them before using them as they may indeed change during development. By all means test and play around with the newer properties (because that's how bugs are found and that's why they are vendor prefixed) but just don't use then for real unless its your own personal hobby site.
CSS3 is being implemented piecemeal unlike css2.1 so browsers will probably never have full css3 support but certain properties will be safe cross browser. You can use sites like caniuse to gauge how much support there is for a certain property.