Should sujood as-sahw be performed for both integral and obligatory parts of the Salah?

There is a fundamental difference between missing a rukn and a wajib. If you forget a wajib thing in Salah, you can compensate for that by doing the sujood as-sahw. As for the rukn, you have to do it otherwise your Salah will be invalid; whenever you remember that you missed a rukn, you must do it.