It depends on what "for real" means to you?
It would be nice if every het had them and no non-hets had them but I think just because that isn't the case doesn't mean there is nothing to it and it can't be useful.
I've not heard of any actual studies. I did read a post by Corey Woods once estimating that 80% of het pieds had the markers and another post by someone who claimed that Peter Kahl told them that about 70% have them. I also saw a post that reported that Peter had a large number of markered possible het girls fail to prove with an individual pied male but I didn't see an answer as to where that pied male came from and if he ever produced pied offspring.
If it really is seen in 70-80% of het pieds I would say that there must be something to it even if it isn't a sure thing.