Having done both (my AD decided to make an 'arms down' model for our last game), I have had many more deformation problems with a T pose than the other extreme. Weighting with the arms down was a real bitch (especially since it was an anthropomorphic dragon character and practically self-intersecting), but the deformation…
You also need to take into account bone LOD/culling (if your engine supports it). In bone culling you start losing the end of bone chains based on distance. If your model is in more of a relaxed starting pose you can actually start LOD'ing much sooner. This is particularly true with hands/fingers. It helps against the flat…