I have watched numerous "instructional" videos on how to change a tire and a huge mistake I'm seeing - and it's shockingly common - is either ignoring the location of the valve stem or misunderstanding its importance in determining where to start dismounting or mounting a tire. The goal is to not have the tire bead trying to occupy the center well in the location of the valve stem - the tube is fixed in place by the stem, and can't move out of the way. The correct procedure is to start at the valve stem while dismounting a tire and work both ways around to the opposite side. Start opposite the stem and work your way around to it when mounting a tire. Following this protocol will dramatically reduce the odds of a tube pinch or tear during tire dismounting/mounting.