To be honest with you, I wouldn't worry too much about opening, or closing your shoulders from shot to shot, or for one spare over another. The thing to do is find which works best for your game, and perfect that. You will naturally open your shoulders if you are "bellying" your shot, and they will be more closed if you are going up the boards with your shot. One of the things I've learned over the years of bowling and coaching is the K*I*S*S principle. It stands for Keep It Simple Stupid. I'm not refering to you as stupid, as you are obviously a very intelligent person, please don't misunderstand me. What I'm referring to is that the less you try to think about, the easier it is to duplicate your shots, which in turn lead to higher scoring. Instead of thinking of all the things you need to do to make a quality shot while on the approach during your pre-shot checklist, let your sub-concious take over and think "quality shot". Your body knows through muscle memory what it needs to do to make a good shot. Let it do the work for you as it is much stronger than your concious thought process, not to mention quicker. Ever notice how once in a while you may be sending a shot a little wide of target, and you "Magically" hit the shot a little harder to get the ball back, or vise-versa, kill the shot a bit if inside of target, that's your sub-conscious doing the work for you. The biggest thing is to RELAX! It will help your game, and you will have more fun too.
Keep 'em falling...