No.

Having Annex "a" support, gives a lighter computational algorithm of "a". The GW do select that automatically, depending on the platform and the "dsp complexity" chosen, where applicable. Basically, you don't need to worry about it.

Annex "B", is a sophisticated VAD scheme embedded in the codec, that cannot be disabled. don't select g729br8 unless you really need to save bandwidth (not much people like VAD anyway).

Annex A & B can also coexist.

Hope this helps, please rate post if it does!