Place your self in the committee's shoes.
How can you legitmately differentiate applicant's?
A national standardized test is a very good way. Therefore, USMLE Part I scores are very important and are used as cutoff scores, just like they have been for the past applications you have submitted (ie. SAT, MCAT). Step I is undoubtedly a super important part of your application to any residency program.
Now, about part II. Some applicants have taken it, and some haven't. It's generally true that people score within +/- 15 points of their USMLE Part I score, but it's not the rule. Also, many programs don't use the Part II score, because there is no national "law" about the timing to take it. But, step into the committee's shoes again. If you have 2 equally weighted applicants, from the same school, solid interviews, etc. EXCEPT one took part II and did reasonably well/passed.... then wouldn't you choose that applicant? It's essentially data collection, and the more data they have on you, the better they can make an informed decision on you.
But, it's up to you to present the data in a positive light, and to make the data as outstanding as possible. Some of you may need this extra data point (Step II), for others, it may just confirm already known facts, and for a rare minority, it may hurt you.
Therefore, the generally accepted advice is to kick so much ass on your Step I, that you will not EVER have to worry about part II, until ... it doesn't matter to anyone.
Take that last bit with a grain of salt, as well. Because, some fellowship determinations, especially in the surgical fields, use Step II. Also, if you decide to jump ship in the middle of your specialty training and train in a different residency field, then again, step II may come back and haunt you. So, prepare for step II, at least a little, whenever you decide to take it.
My Summary: Crush Step I; Take Step II at YOUR whim, but prepare for it.