I’m sure you can find lots of different opinions for this. Some may start from a more natural, arms almost down-pose because then it is easier to model and sculpt a natural shape. Then they may change it to T before rigging. Some keep to T-pose for modeling, rig the character and fine tune the mesh when posed more naturally.
Modeling pose arm-angle you choose depends on how you plan to animate. Most of the time we keep our arms at lower than 90-degree angle, so it would make sense to model in a pose that suits that intended range of motion. Many favor pose that has arms at about 45-60 degree angle out from body sides.
Especially when you model in lots of detail, muscles, tendons and all, it is helpful not to animate the model far from that modeling pose. Rigging all that detail to deform just right in all poses is a pain, so we ease our way by modeling to a pose that’s roughly in the center of the inteded motion range. Saves us some rigging trouble. If you do go for high detail model and also want natural deformation, muscles and so on, AND wide range of motion, I would go for T-pose and then go crazy rigging it.
I tend to favor T-pose ofr the ease of rigging and the widest range of motion it gives. I like to stress-test my characters, make sure they can do extreme poses even though I may not ever need them.