Generally it's better to work on one issue/key at a time. Example, if I try to work on keys 1 and 3 for example, it's harder for me to concentrate on key #1 rather than if I just worked on key #1 by it self. This allows me to get that key down faster than skipping around to other keys/issues. Once I can swing and not move my head all over consistently, I'll then move to key #2, rinse and repeat, and so on.
I'm sure Dave, Erik, Mike and all the instructors have found that focusing on one thing at a time produces better results, I know it did for me, and I still practice Key# 1 all the time. Hope that answers your question.
Good post, very much agree. Good thread about how to go about practicing.