I've just always done what they have been most comfortable with. I even remember during the time between having my 2nd baby (where in hospital it was find to lay them on their side) to having my 3rd baby, the *rules* had changed and that they were to only be layed on their back. I asked about choking and they said that on their side they have a tendency to choke more, but if on their back, the vomit could come up, but just run back down their throat and not into the airway. I just did it while at hospital to please them LOL and then once home did what bubs was happier to do - even when on their back they always turned their head to the side so I just put them on their side rather than them get a crook neck LOL.