As a general principle, which you can apply, you will have agonist/main moving muscles, and antagonists/opposite muscles for any movement.
When the back muscles are the antagonist, it is important (usually) to keep your shoulder blades pulled down and back, hard, throughout. They should be stable.
E.g. Military press: DO NOT gain range of motion by allowing your upper back to 'float' or get loose. That will eventually injure your shoulders. Instead, push up against a constant downward pull from your scapular area.
When the back muscles are the agonist, the answer depends on whether you want to train lats more, or midback (eg rhomboids etc) more. For midback, I let go of the contraction/ release the shoulderblades, then pull back with that area, eventually finishing the move with arm pull. When this feels natural it will still be two stages, but will occurr as one fluid motion.
E.g. Bent-over row: after setting up, round shoulders forward. Then, squeeze shoulderblades down and back. Then, pull to abdomen/wherever with your arm pull.
For lats, I have found anecdotally that not retracting shoulder blades allows you to target lats more. Just pull with your 'armpit muscles'. For pullups that may mean you alter your angle of pull.