Too many people think in absolutes. Either they want the elbows flared out to 90 degrees, or they tuck them in hard by the sides. Neither option is great for your shoulders.
With the elbows flared excessively, a ton of stress is placed on the shoulder joint. It’s also an incredibly disadvantageous position biomechanically, so not only will it feel like crap, but you’ll perform like crap, too.
On the flip side, tucking the elbows in hard to your sides isn’t a great idea, either. While most do this with the intent of sparing the shoulders, what ends up happening is that this excessive tucking causes the humerus to glide forward in the glenoid fossa. In normal people talk, you start to get an “owie” in the front of your shoulder.
The best choice is to find a balance between the two extremes.
Make a 45-degree angle with your elbows, or simply “make an arrow.” This cue works like a charm for shoulder health and performance.