Gordon's recommendation only applies if your tap water has high alkalinity. If its low in alkalinity, then adjustment with an alkalinity producer such as lime or baking soda is OK after mash in.
Phosphoric acid is perfectly fine for pH adjustment. There is a myth circulating that adding phosphoric acid causes more calcium precipitation in the mash. Since wort typically contains on the order of 1% (which is 10,000 ppm) phosphatic compounds, adding a 100 ppm of orthophosphate ions to the mash is not going to appreciably change the amount of precipitation of calcium in the mash.