I think that rear parking sensors are not necessary because you can calculate the distance by software out of the backup camera image. Parking sensors in the front are nice to have. My opinion in general is that parking sensors are most of the times rendered useless (personal experience), a back up camera is just fine if it provides a good picture.
Even on narrow roads in Europe, and I'm from a town with really narrow roads you can drive with a 2m+ vehicle without parking sensors if you know your car. Parking sensors at all are Useless if the spot you want to park is to tight for your car, because most spots are only 2m in width.