The nipple may still freeze if you do not put a return line back to the the supply. This could also depend on how long the pvc pipe is. You could also add a heat tape to the pipe. I just use a 30' heat tape wrapped around my 55 gallon drum and along the supply line to the pvc pipe that my nipples are mounted on.. It has worked well so far... Only problem is the birds like to eat the foam pipe wrap.
What is heat tape?