Dartmouth requires step II before you match. (New Hampshire has a law requiring it to be passed, so therefore they require you to have passed it before you are ranked). At least, they act like they do. When I sent in my step II score to them around january, they replied with a message about them being happy because now they would be able to rank me.
If you are an IMG, step II is more important than if you are an AMG. But, either way, it can only help you to do well and submit this before the ranks are due.
Step II is only a gray area if you don't do so well. I know it works out better for many to take it later in the year, but unless your schedule is so busy that it prevents you from making any reasonable studying effort, you should take it before february. It is very nice to have your score back and be able to submit it to programs.