The undervoltage the ballast experiences during starting is not the "greatest" for it. Will it hurt it? Maybe in the long run.
I put in a separate switch to toggle the low beam on and off, although it burned out after three years. The choice ends up being yours - 50/50 either way.
Also - I wouldn't use the HID for the high beam as you loose the "flash-to-pass" functionality (HIDs have to "heat up" first)...