Adding more GH or Mg will not hurt, but it may not help either.
Tap water tends to be pretty stable with GH, after rains, if you rely on rainwater, surface waters, the levels will drop in the winter/spring, climb back up in the summer Fall if you get a lot of water in the winter/spring etc.
Tap will vary. Generally, plant might only ever need 1-2 ppm a week of either nutrient.
Which is not much.