There are several pros and cons to weigh when deciding when to take it. The first thing to know is what requirement your future residency has as well as the consequences of not having it. Some places require it by the end of PGY-1 vs. 2 vs. 3 whereas some don't care as long as it's before graduation, but then you need Step 3 in order to moonlight.
Taking it before Intern Year:
I wouldn't necessarily recommend this. It would take someone with a lot of motivation to do this. All those radiology/anesthesia rotations have probably got you so deconditioned from medical knowledge and there's really no rush taking it so soon.
Taking it early in Intern Year:
This is what I'm doing, but only because my schedule is painfully backloaded and I was given two weeks off in November. The benefits are getting it out of the way. I also took CK very late so my Step 2 knowledge isn't that far away. This is probably also ideal for those in non-IM specialties not doing a medicine intern year (OB-GYN, Peds, Surgery and its subspecialties) because you're never really going to get a foundation in medicine (70% of the exam) anyways and it gets it out of the way in case you need to do research, moonlight, or study for the ABSITE in your specific field.
Taking it late in Intern Year/Before Second Year:
This is probably what most IM residents do. With IM or EM residency, the exam should be easier by then. That being said, if you don't have a break then (like me) it becomes hard and the more time you waste worrying about Step 3, the less time you spend doing research/moonlighting.
Taking it after that:
Don't see why you would need to do this outside of procrastination. Your knowledge may improve marginally...also you're going to start to forget some of those Step 1 factoids that still show up on Step 3. By this time you want to be spending your free time doing research/moonlighting/etc.
