Yes, it is indeed a good technique, as long as it is done the way you described. Too many people use the idea of "letting go" as a technique of forcefully fighting against their emotions and getting rid of them, like their anxiety or whatever other problem they might have, but that is just suppression. Feeling angry? "Oh, just let go, there's no reason to be angry and yada yada yada you shouldn't be angry." That type of stuff. The correct way to do it, like you describe, I would describe as an openness to all experience, including the most painful emotions like depression. Which is pretty much the same as "doing nothing". With this attitude there isn't really any need for a "letting go" technique because there is nothing you want to get rid of, if that makes sense, so it would be pretty much useless to anyone who want to manipulate their experience. Because why would you? That's at least how I experience it.