You likely will not be testing GH, once you know the tap water GH. If you need to adjust parameters for some reason then a GH and possibly KH tyest is useful, but this is not as easy as it soounds (adjusting parameters). If you do like most of us and ascertain the GH (especially) of your source water, this is not going to alter much if at all in the aquarium (unless you target it, that's another story) so you just acquire fish suited to your water GH.
The API Master Combo is pH, ammonia, nitrite and nitrate and these four are or can be/should be tested periodically, or frequently, at the first sign of trouble, etc.