Few guys at our club did some testing with it.
They shot many shells on paper to patern it and they noticed the wads, most often then not, wouldn't be in the middle of the partern.
So... Is it helpfull ? maybe but not a 100% reliable.
I wouldn't think they would have to be in the middle of the pattern, as long as they are in it. If its in your pattern the clay breaks and you don't really car where the wad is, its when you miss you'll be able to see if your behind, above ect.