As prevalent as the practice of leaving the placement (or color) open-ended is, doing so isn't a good idea: people'll be hesitant to rate your design highly because they can't be guaranteed that the placement they prefer will be the one selected should the design get printed. Instead, get placement suggestions in the critique area before you submit your design.
That said, the the off-center placements are ideal but all the arms would have to stretch to the edges of the shirt for it to work. With the design as it stands right now, though, the center-positioning is the best imo.
I design it in two version, white and black version
Hope you like it
Thanks