Do I need to flatten my whetstones, and if so, how often?
Unless you are sharpening single-bevel knives, such as a traditional yanagiba aka “sushi” knife, kiritsuke, etc., having a perfectly flat whetstone is not absolutely necessary. Does that mean that I’m against flattening your whetstones? No. I’m simply against the notion that you have to flatten your whetstones before or after every sharpening session in order to get a sharp knife. One can flatten their whetstone(s) as often as they wish, but ultimately, it’s about sharpening skills and knowledge that will result in the best edge for your knives.