Somebody has to test and debug your app on actual PPC hardware.
Our Xcode-supported unit tests transparently run three times -- once for x86_32, once for x86_64, and once for PPC. The PPC run occurs within Rosetta (ie, emulated).
If the tests pass, we can be reasonably sure everything is A-OK. In addition, we can do local regression testing under Rosetta (but it's rarely necessary -- usually everything just works).
The only native PPC testing we do is integration testing once we reach the end of the development cycle.
Neither the compiler nor the emulator are perfect. Somebody has to test and debug your packaged app on actual PPC hardware.