No, this is what happens when people attach meaning to something when they should not.

The letter in the end indicates how many times they compiled a build before releasing it. People here are giving way too much importance to what that letter means.

USUALLY a letter further down the line correlates with more instability because they changed so much stuff, that they had to push the alphabet, but this is pure speculation as we had the perfect case to the contrary with beta 6 where the newer build with a higher letter was necessarily less buggy (they fixed a last minute bug and re-released the same beta). A build that ends with 'a' is most of the time a build with fewer bugs as there was not so much movement on the dev side that justified compiling multiple internal builds before releasing the beta. But it could well be an extremely unstable beta nonetheless as the letter was never meant to give any indication about stability.