Will one power adapter work with all shipping MacBook Air and Pro as of March 2013?
The 85W MagSafe adapter should do the trick for you.  It's compatible with the original MagSafe design and provides enough power for all of the Intel Macs up to this point.

Picking up a 45W or 60W wouldn't save any money and though they would both run all of the Intel Mac laptops they be insufficient for powering a larger (15" / 17") Mac back to life since they require 85W.

Should Apple move completely away from the MagSafe design to the slimmer MagSafe 2 design you can add the adapter.