I think in a reef tank, it is a good thing to test for them, especially if newer to the hobby. I test mine once per week.
I use the HANNA checker, which is easy and convenient. However, I also like the red sea PO4
test kit since it is easier to read than the salifert or API tests. It is slightly less subjective.
kits aren't the most accurate kits, especially the ones we use as hobbyists. However, I think more important is to just get an order of magnitude of how much PO4
is present - the EXACT value is not as important. Most of the kits out thee will provide a good rough order of magnitude for you.