As an IMG, I'd give yourself some margin of error. You can't afford to do poorly on the Step 3 (as in fail the thing). Will a low pass hurt you? If it's a residency program on the fence, I think that it could. You are at an ENORMOUS disadvantage being an IMG, and you need absolutely everything working in your favor to get a PM&R spot.
Fellowships typically recruit HIGHLY from within...so your steps aren't going to matter near as much if you did well in residency, have recommendations from the right people, did the right things to demonstrate interest (research, conferences), and showed that you will be a competent fellow. If you are sold on a particular fellowship (which I would discourage)...I think that it's pretty important to get the right residency. There are MANY fellowships that won't even look at outside applicants until they have reviewed their own inside applicants. The single most important factor in getting the fellowship you want is landing the right residency. That's not to say that you can't get a fellowship coming from a small PM&R program without an inside fellowship (especially if you are interested in either Brain or Spinal Cord)...but you will be at a disadvantage if you are trying to compete against inside applicants.