Many people in the industry believe that arms out at 45 degrees (preferably with palms facing down) and the legs about shoulder width apart is the best way to get relaxed posing out of a figure. The T-Pose, as used by many Poser figures is still popular though and is a carry over from a time before weight mapping was a common thing. Depending on the studio you work at, some places will require T-Posed characters while others won't or won't care and that's all based on the rigging tools or person doing the rigging.

As for Poser, one of the reasons I was told we still use the T pose is because it makes working with Walk designer a little easier (no arms crossing over or sticking out oddly). There's likely other reasons too.

I personally like to model with the arms in a 45 degree position and then reposition the arms for its final T-pose position in Poser.