I never got to actually push with any of my babies due to emergencies, with my first I spent most of my labour on my side. I had wanted to be crouched in the tub on my knees but my blood pressure was too high. I think, generally though, it is what is most comfortable for the mother.. and can sometimes depend on the situation. Some labours allow you to be more mobile and flexible than others due to your vital signs, the baby's vital signs, whether or not you've decided to have an epidural (then you do need to remain on the bed, most likely a lot of women in the videos had one.)