Ah right I see where you're coming from, low pH due to H+ ions. That's not a problem here (and in other hard water areas I suspect), since the tap water starts really alkaline due to the hardwater content - well above pH8 (it was about 8.6 when i lived in Wimbledon, a bit lower in Surrey).
Hence the de-ionized water may be a bit better than tap as it will be closer to neutral. Also, the water is de-ionised a split second before it hits the car, it then runs off and aqua wax is sprayed on...
Tap water is so alkaline in hard water areas (being most English cities and river basins) that I fill my fish tank with rain water from our water butt. That is pretty much bang on pH7 according to my indicator kit. As the tropical fish I keep prefer pH6.5 average, I still add a little acid to the water.
Brat Pack no.8