For a lot of aspects of cannabis-growing? – There’s no strong consensus. Most recommend at least a slow-down of nutes before harvest, but some disagree.
Think about other things that grow on trees. How much nutes do you taste in apples, for instance? Hey, there may be some there there with the “nute taste” thing, but if there is, it’s not obvious to me. Personally, I just make sure I’ve no-nute watered twice in a row before a harvest . . . kinda to split the difference.
Same with flush. A range of opinions. I flushed all the ladies once, but intellectually in darkness, since I wasn’t measuring ppm! The point of flushing is to more ably control the environment. All the latent salts create a wild card that can make your grow less predictable. But ultimately IMHO . . . the best “meter” is your plants. If they’re happy . . . don’t flush.