It depends on how much you care about perfect grades and how much you care about what others think of you. If you recognize that a big part of your clinical grades is determined in a subjective manner by what your attendings (and in some schools, residents) think of you, then you might also recognize that this means a big part of your clinical grades can be out of your hands depending on what situation you might be in. I've already found that some attendings think very highly of me (and openly say that) for little reason in my opinion because I gave them very little to evaluate me on. One attending only really saw me for a few minutes every morning and only saw one or two of my patient notes, yet apparently this was enough for him to tell me that I'm several standard deviations above my peers. Counter that with one of the evals a resident gave me in which he gave me 3 out of 5 for everything he evaluated me on (which is oddly designated as "good") when I really got to know this resident, worked my rear off with him every day for several hours every day, and actually got along with him quite well and received a lot of excellent verbal feedback. When it came right down to it, his 3 out of 5 eval earned me only 60% of almost 9 possible points he was allotted towards my final grade in that clerkship. What if several others evaluated me the same way for that clerkship? I'd be screwed if I had my heart set on pass with honors.
Obviously you also have to keep in mind that these shelf exams can be somewhat random and at times do not correlate very well with what you know or how hard you worked.
My advice to you is to determine how much you really care about perfect grades and how much you really care about what people think of you. I'm not saying you should rub people the wrong way, but you might be less anxious and much happier if you take some of that pressure off yourself. Do what I do and think to yourself: "I'm going to work hard, I'm going to learn a lot, I'm going to have fun, I will not be scared of questioning people (tactfully of course) because I will not be worried about how they will eval me, and I will not have regrets even when the grades are in because I know at this level it is far more important what I get out of these experiences than what some stranger says after the fact what he/she thinks I got out of them"