As far as shelf exams go: no, they aren't listed on the ERAS. I did have a one page computer supplemental app for one program and they asked for shelf exam scores, but they had a box "my school odes not give shelf exams or does not relase scores" and I checked that. My school converts our raw exam score to a curved grade percentage, so I didn't know my scores they way they needed them.
For step 1 and step 2: the step 1 is more important than the step 2, but doing better on step 2 could help and won't hurt, so you might as well try. It won't make up for/cancel out the step 1 and isn't "worth as much" however. That said, I have a friend with below ave grades, low sep 1 and improved step 2 (I don't know the numbers) and he matched gen surg, so it seems like it helped him!