It all depends on what system, and/or what levels you want from your tank.
First decide what levels you want in your tank of each nutrient. If you are unsure, it's probably best for you to try a dosing schedule like Tom Barr's (plantbrain) "Estimate Index" that makes sure you have excess levels in your tank so you don't have to worry about this.
If you have decided on the levels you want, you can test as much as you like! The Perpetual Preservation System (PPS) is another method, that tests on occasion to see which nutrient needs dosing and you dose up of that one in particular.
If you want to make your own schedule, it is best to get a feel for how fast the plants use up those nutrients, so as to know how much and how often to re-dose.
Testing before dosing will tell you the current levels, and show any defficiencies. Testing after will confirm the new levels in the tank.
It all depends on how much testing you want to do, how fast you want to go through a test kit, etc........ personal preference.
If you don't develop a schedule for dosing, I can guarantee you will go through a lot of test kits trying to find the levels and when to dose "on the fly", and probably will end up with a tank out of balance due to small dosing/testing errors.
Do a search for the methods I mentioned and read up about them.