I think if I vibe w/ what zeb said, and having taken the Step 1 myself, you really need to ask yourself how awesome is your memory. You get about 2 months over the summer to study for the Step. And let me tell you, the stuff you study week 2 is NOT as "fresh" as the stuff you study week 7. Granted, you can jog your memory w/ far less effort in subsequent efforts, but there really is something to saying that the most brains do have a finite "RAM".
I had zeb's attitude. I studied for the Step from day 1. Telling myself I wasn't learning things to pass an exam, I was learning things for the Step. When we finished anatomy I went and got some NBME/USMLE anatomy questions. And did great.
However, once med school pours it on, you DON'T have time to go back and remember everything in anatomy and genetics, on top of trying to keep up w/ the sheer flood of information from pharm or patho. There is NO point in sacrificing your immediate study tasks, at risk of doing poorly in your coursework.
That said, it is to your benefit to learn the stuff you have in front of you really well, cuz reacquainting yourself w/ it during Step prep is easier than had you tried to do the minimum to pass your exams, and studied for Step concurrently.
I mean, if you find yourself w/ free time in Basic Sciences, it won't hurt you to try to start your UWorld or Qbanking. But the moment you find yourself not fully understanding a concept in your concurrent classes, you DROP what you are doing Step wise, and address the problem stat!