I'm righty and unclip right first. I've no better rationale than it's just what was comfortable for me. I always start off with my left foot clipped in.

My boyfriend is a lefty and he generally unclips left first. His sister-in-law is right handed and she unclips left first. His brother...I'm not so sure.

I've noticed that if someone is in front of me and they unclip I will unclip whichever side THEY unclipped even if it's left. Kinda strange...

My BF (and eternal bike tutor) encourages being comfortable with unclipping from either side because there will be moments when you can't unclip your preferred side first so you should be able to unclip well from both sides. Sounds great in theory but I've had quite a few sloppy stops trying to practice unclipping my left foot first I'm still trying though!