That's a good question. There was a study done a while back that showed that students with some clinical experience perform better on the USMLE Step 1, but I don't know how much better. Also, I think residency programs just care that you did take Step 1, not necessarily when.
That being said, there are a few considerations you have to take. First, does your school have a requirement that you have to take Step 1 by a certain time? Second, did you get any clinical experience during your preclinical years? If you did get some (for example -- case based discussions, PBL, ...), then I think that's more than sufficient clinical experience to help you on the exam. Third, does your school give you time scheduled in to study between the end of your preclinical classes and the start of your rotations? If so, that's probably the best time to prepare for Step 1, since you will have dedicated time to study.
In my opinion, I would NOT want to postpone Step 1 until clinics have started just to get my score up maybe 5 points. It's a pain in the a$$ to study for, and you will be SO HAPPY it's over with if you take it earlier. Clinical rotations have a way of tiring you out a lot so that it's very difficult to study diligently during them. I'm having similar issues trying to study for Step 2 right now during my rotations. You may even risk getting a lower score on Step 1 by waiting until clinics have started because your time will then be split and you will lose your dedicated study time. Not to mention that you will likely forget much of that crucial basic science information during you rotations.
Hope that helps!