we check two things regarding the brakes, in the old models (only one in the new ones)
1- the airplane slows down when you press the pedals
2- normal brakes are "in charge", rather than alternate brakes.
So it is better to wait until you are already taxiing to press them and check both things, rather than check one before and another after initiating taxi.
Agree there is no need to stop the airplane, but some people hardly presses teh brakes and there is hardly any noticeable braking action... I like to feel it, so there is no question that we have brakes.
however, I have a question regarding the system (in old models):
if i understand the system correctly, when during normal taxi we press the brake pedals we are pressurizing both the green hydraulic and the yellow hidraoulic lines, right? so every time it is the automatic selector valve which gives priority to the green (normal) brake system. When the pedals are untouched, there is no system "in charge".