True Stigmatas are evidence of Jesus' Passion; however, they can not be taken as a good source to determinate where exactly, or how exactly Jesus was nailed on the cross, for the simple reason that even true reliable Stigmatas show wounds in slightly different parts of the body.
For example, as Gift have showed us, even on the same person, stigmata signs some times do not appear on the same spot once they have gone and come back as in the case of Theresa Neuman which show some wounds in the middle of the hand, then on another picture, the wound is showing a little lower section of the hand.
St. Padre Pio wounds generally show in the middle of his palms, on the other hand, Fr. Sudac show wounds in a very low part of his wrist. However, for the reasons explained above in this thread, neither shows the 'exact' spot where Jesus was nailed. Why God show different spots on stigmatas? why He doesn't show the 'exact' place where Jesus was nailed in stigmatas? - in my opinion Stigmatas is for a remainder of God's love for us, of Jesus' passion, but it is not for historical research - but in general it's a mystery totally known to God.
Another important fact to mention is the following, Jesus' crucifixion is the most famous - Yes; however, His crucifixion was not the first. Crucifixion dates back from about the 6th century BC. Crucifixion was in use at a comparatively high rate among the Seleucids, Carthaginians, and Romans from about the 6th century BC. http://www.britannica.com/EBchecked/...83/crucifixion
So, those executioners, the Romans who nailed Him, were EXPERT in crucifixion, they were doing a job they have been doing many times, and Jesus' crucifixion, was just another one in their list for them. They were no 'novices', they Knew what they were doing - in the sense that they were specialists. They knew that nailing Him thru the anatomic wrists bones of the hand was good enough to hold Him on the cross, and again, as St. Bridge of Sweden saw in her vision, she very specific says: "they transfixed His hand in the part where the bone was firmest."