Ugh, I just realized you were talking about the actual step. I'm not qualified to comment on that. In case you missed it the first time, find the Micro slides by pholston and know those.
In terms of the shelf:
I guess it depends on your learning style. Definitely know FA cold, and if you HAVE to choose between the cards (yeah, those ones) and MMRS, go with the cards. You can do them in a day if you already know a fair amount, but obviously, the more time you have the better. I annotated mine with mnemonics and stuff from the Qbanks and Lippincott Q&A over the semester. I keep hearing that Kaplan's Micro is their best from the M1 subjects, but it didn't work for me. Obviously, the best resource is the one you're actually going to use. So find one that works for you, and stick with it.