I tend to do palms down. If you stand with your arms straight down, palms inwards (like a soldier at attention) there is no 'twist' evident in the forearms and feels relaxed. Swinging the arms straight up and out to the T pose position therefore has the palms down. On the other hand, if you stand in a T pose with palms forward, and swing your arms down to your sides without rotating your hands, there is significant twisting apparent in the forearms. I believe this is due to the twisting action of human arms being shared between the shoulder joint and the forearm bones when the arm is straight out, but when the arm is down the shoulder joint is not able to contribute as much to the twisting action and the forearm must compensate if the palms are to point forwards.
