A few operators I dove with had requested that you hand up your fins, but the way I was trained is most comfortable to me and I've never seen an operator push the issue if I keep my fins on my wrist. Mainly, they're just concerned with not having people try to climb the ladder with fins on when the ladder is not designed for it (And I've never had the misfortune of climbing a ladder designed for it)
If my fin straps are around my wrist, I always have them handy but out of the way. If I lose my grip on the boat, I can just slip them back on. Not so easy if my fins are already on the boat.
Even in relatively rough water, I don't have a problem with them 'slapping around'. If I'm worried about it, I further loosen the straps while I'm still in the water and shove them further up my arms. Any conditions where my fins *might* 'slap around' are just more justification for me to keep them with me in case I come off the ladder.
If my fin straps are around my wrist, I always have them handy but out of the way. If I lose my grip on the boat, I can just slip them back on. Not so easy if my fins are already on the boat.
Even in relatively rough water, I don't have a problem with them 'slapping around'. If I'm worried about it, I further loosen the straps while I'm still in the water and shove them further up my arms. Any conditions where my fins *might* 'slap around' are just more justification for me to keep them with me in case I come off the ladder.