Subvocalization is an issue that has been talked about on this board on and off ever since the beginning of PhotoReading's existence, as well as the knowledge of speed reading.
Perhaps there could be something addressed in this with something like a 4th Edition of the book, or maybe just some kind of insight from LSC.
I think that it really does not matter. I don't really use it, and with PR, you eventually get to a point where not much acitvation is necessary anyway.