Once you have a cycled tank you can drop the nitrite test kit.
Ammonia and nitrate are always useful, but much less so after the tank is established.
Once you have fish and corals in the tank you can start with Ca, alk and Mg tests. But unless you have a lot of lps and sps corals, these may not change much except when you do water changes.
Phosphates... I gave up checking it years ago. The algae uses it so fast and kits are terribly unreliable, IMHO.
pH... it is what it is. It may be useful as a tool if things are going wrong in the tank, but as a routine test it's not very helpful.
All marine test kits are only close approximations of actual water parameters. And each individual has the ability to have an affect on the results, i.e. we both do the same test, on the same water, with the same test kit and there is a reasonable chance we get different readings... probably close, but different.