If I am testing, I test my tank before I do a water change, so I know how much water to change if it is a fish only tank or how much nitrate to add if it is a planted tank.
I don't ever bother to test after the addition, once I know I mixed the formula correctly. So, when I was first mixing the phosphates I did test after, to check my numbers, but never after that.
Member AGA, HAS, NASH
110G, 10 adult discus, barebottom with plants on wood
58G, 3 adult discus, barebottom with plants on wood
59G thickly planted tanks, rummey nose tetras, kuhlies
300G goldfish pond outside