I used to moonlight as a harpsichord tuner, and I didn't know anyone in the field or any piano tuners with perfect pitch. As I understand it, it doesn't help at all because perfect pitch is not precise enough to tell 440 Hz from 440.1 Hz for example, while you can do that easily with a tuning fork (once you learn how to listen for the beats). On the flipside, unequal temperaments - which are frequently used if you are a harpsichord tuner - are hell for people with perfect pitch who listen to only equally-tempered music. When you "merely" have relative pitch, unequal temperaments can actually be nice.