If you have a complex design that breaks when font size changes, you'll want to use px for font-size. IE has reserved this measure not to change when the browser "view" is used to increase/decrease text size.

Ems are really nice to keep design from breaking, as long as you use them for vertical box element measurements.

I wouldn't advise anyone except an advanced css user to use %. The inheritance rules are just one more variable to mess you up.