You don't HAVE to buy the HST ruler to make HST's. I have seen Jenny make the HST's that way, and I do somewhat agree with what your shop owner says, and this is the main reason I don't make HST's this way, but instead cut my 2 squares 1" larger then the size of the HST I want, then trim it down, but if you want to make it Jenny's way, I would suggest that you starch good and that should prevent any trouble with the bias.