I don't think it makes a difference
Although if I'm wrong, I hope someone will correct me.
If I'm not mistaken, in one of the tapes that has plie squats with a barbell (Tough Tape? PS SLA? MIS?) the instructor says you can either have the weight in the front or on the shoulders. In either case, your legs have to bear that weight, so do whatever is comfortable for you (if you can find any comfort whatsoever while doing plie squats!).