Depends on how you did on Step 1. If you rocked Step 1 then you will likely be granted a decent number of interviews without a Step 2 score, so you could put it off a bit. But competitive programs may appreciate a score before it comes time to rank you (interviews usually wrap up end of January, with rank list certification deadline end of February). They also want to make sure you didnt fail, which is unlikely if you did well on Step 1 but not unheard of. Some schools require passing Step 2 for graduation. If you dont graduate, you dont go on to residency that year.
If you didnt do so hot on Step 1, you may want to show how you rebound from adversity and take Step 2 earlier - before interviews are granted. The bulk of interviews are usually offered mid-September mid-October, with another spike soon after November 1 coinciding with the release of the MSPE. Take into account the amount of time it takes to get your scores back (about 4 weeks), and obviously take into account how prepared youll be to actually sit down and take the test.
Relatively speaking though in PM&R scores arent as important as other factors (rehab experience, letters of recs, etc.). Ive interviewed plenty of candidates with less-than-mediocre Step 1 scores and no Step 2 scores, but other aspects of the application were strong. So it probably doesnt really matter. If it were me, Id just as soon get them over with as soon as possible. One less thing to worry about.