Filling below and first 6" or more above pipe with sand can buffer ground movement if that is a problem with your soil when it changes moisture level. You certainly don't want rocks in contact with a water line. What purpose did the plumber say the rock would serve?

English was his second language. I think he may have meant sand - he couldn't find the word and said "not quite gravel" and I suggested "pebbles?" and he nodded, Sand makes a lot more sense. Thanks!