That's not what I understood the letters to mean. It was explained here in the forums that the letter at the end of the number signifies how many times it ran through internal QA before the build was released outside Apple.
And, historically, it's been my experience with iOS that the higher number of retries after a build has been made, the less polished it is as a result. A good example is the Safari bug we found earlier today. It wasn't there in Beta 2, which was a (d) build, but it showed up in this one.
This build had to go back once more through the QA cycle than Beta 2.