I would try to avoid "b" at stages, because it can sometimes be innacurate. For example, London Control instructs you to climb to FL130, if you are still at the local QNH of which you went through the transition layer with. Then your altitude will be wrong. You should allways change it when passing the real TL to qnh 1013 - or mercury 29.92.
For example..
Climbing out of heathrow, TA = 6000ft, TL= 060 - So you should set the Standard qnh when passing.