Depends on your water and maintenance habits, fish load, etc. You'll need to at least test your tap once to know what you have. If KH is high, then pH will be stable as long as you have some sort of water change routine going on to replenish buffers. And even if you have to replenish buffers like I do, once you get into a routine that you know works, it's rare that checking is needed.
Now as for GH, it's not nearly as important, at least to me, but still good to know your tap parameters. As long as that's in a good range, then you should never really need to test for that, at least not very often.
So, anyway, you don't want to just know pH and ignore KH, that wouldn't be a good idea. If they were going to bundle two tests together it should be pH and KH. GH and KH have little, if anything, to do with each other.