I know someone had posted a chart on here once that i repped him for showing the different Ph's that plants will uptake nutrients better at. In the early stages of flowering you want the ph to be low 6's and as you go longer it slowly goes up to around 7. It really changed the way I was watering my plants over the course of their lives in the flower chamber.
But to answer your question. I would never put unchecked tap water on my plants. you can still fuck em up this close to being done.