Common rule is T pose for beginning riggers. It align the arms and hand with world axis thus making it easier for beginning riggers.
Outside of that, the choice boils down to it depends. Focusing on humanoid characters. If your project is planning a lot of high arm animations like climbing, swimming, etc. then T pose is your choice. If your project is more every day activity like walking, running, most dancing, etc then A pose is the choice. A pose give easier more natural shoulder deformation. With stiff characters like robots or non-flexible armor you want a pose that is half way between the two most common extremes planned for your animation. With animals/monsters you want either rest position(think T-Rex arms) or mid way. Again it depends on the project and your character design.
