The two major things I test for is gh and TDS. After I set up a tank, I either will do a weekly/monthly wc, or only change when TDS is too high. Otherwise I top off with pure RO.
I have an rough guide as to what TDS contains the proper amount of gh that I am looking for since I use SS and Ro. So I take that into account when I do wc.
I have the other tests to troubleshoot if something goes wrong.