Many public toilets here have the taps with sensors which automatically turn the water on for a fixed period when you stick your hands under them, so no touching is necessary.
Others are the type where you press the top to get the water to flow for a short time, in which case I bang the top with my elbow.
I guess if you have to actually touch the tap with your hand to turn it on and off, then at least when you're turning it off your hand is already wet, so any germs transferred back to your hand will be suspended in the water droplets on your hand, which you then remove when you dry them.