"Required" is a very strong word, each individual nutrient has a preferred PH, Nitrogen has a very hard time uptaking in rootzones anywhere in the 5's so I certainly can't recommend that for the veg phase considering Nitrogen is the major macronutrient at this point. 7 makes sense for Potassium as it uptakes at its highest at 7.5. 6.5-7 is generally used because it allows reasonable movement to most nutrients, and restricts none. The reason for adjusting PH would be to encourage to maybe bring out color changes?