Vertical nipples shouldn't leak unless the pins somehow get jammed. With all nipples, chickens dribble water since they lack drinking manners, no matter how many times I try to school them not to leave puddles all over the ground.
The issues can extend to more than just mess. Frostbite from water falling on feet is a real winter danger with these devices. So I've come up with a simple way to prevent this. I've attached plastic ice cube trays about ten inches below the nipples to catch the drip. It's solved all my problems. The chickens keep the drip trays pretty well emptied by drinking from them, too.