Brewno, I have used the same method you follow. It works well and should yield very consistent results. The method that your LHBS uses will work fine too because with all that grain in the pot, the pH isn't going to rise enough to lead to tannin extraction and off flavours unless you actually boiled the grains. But my concern is that this is a rather imprecise extraction method, so your results could vary slightly from batch to batch. If that isn't a concern, either method should work fine.