I wear mine over the right shoulder only; it does not go over my head with one side over the shoulder and one side under the opposite arm. I wear it higher than a lot of people probably. I find that I have more control up higher and can squeeze harder when my arms arm bent than when my arms are extended more straight in a downward angle. (you ought to experiment with that, you arms are at basically right angles when you play sitting, also your wrists should not have to be too bent in order to play--that is a recipe for corpal tunnel(sp?)I think they should be about the same when playing standing with a strap. You will find you have more control like this. It might not "look as cool" though!)I have seen every combination of height and shoulder strap routing. Tim Broussard in our band who normally plays bass is an excellent accordionist also. When he plays my accordion he puts the strap on over his right arm and head. (the strap goes on the left side of his neck and under his arm on the right side)
Bottom line...there's no one right way. Experiment and find what works best for your body build and for your style.
Thanks to all that gave some good tips on using the shoulder strap.
My main problem was to anchor the right side of the accordion so that
I was not pulling my thumb off my hand. I have a better understanding now.
..... Ed