It’s best to take a proactive attitude to soil testing. It makes sense to find out if your soil is healthy before you start planting. That way, you can have full confidence that your plants are going to be as healthy as possible before you start growing them. Some may take a more reactive approach as simply try and solve issues where they see them, which is fine. While this can be less exhaustive, it means you can waste time growing plants in unfavorable conditions. Once you have the right levels, then it's a good idea to check your soil every few years to see if anything has changed.