Fix the major bugs that are found. Test for both bugs and balance. I suppose these both could be filed under “listen to your testers.” It is fine to release live with some known minor bugs; there is a trade-off between delaying content and making a more perfect union. We know that we will need to fix some things after the patch.
I know that changing something by 20% is just a minor tweak to the code, but it makes a pretty big difference if you do so after the new content has been out for several weeks, and it looks bad when players were pointing the problem out on your test boards since the day it hit the test server.