Doesn't matter what pH you are watering with, the medium/soil needs to be at the correct pH, the water will not change that. I'm still doing testing myself but I believe salt (epsom salts used in gardening cheapest I think, seems to works well), amended into or dissolved into water then mixed into soil is the easiest for a plant already in a pot. I believe wood chips work but take a little time to start leaching to bring down soil ph, other stuff as well.
Make growing medium ~perfect before starting any plants. I haven't done this myself in the past but I do now.
From my rescent testing, not much, it seems like water only effects ~0.1pH for every 1.0pH that it is off. And that was enough for me to not care about it, I used to think water pH will have greater effect.