It certainly depends on how your school plans their radiology clerkship. At Iowa, we have a 2-wk rotation that can be taken in 3rd or 4th year. Our program director suggested I take it earlier in my 3rd year, mainly to get acquainted with some of the staff. I'm not so sure I got very acquainted with anybody, having bounced from one department to another every single day that 2 weeks. But, I do feel that having taken it before I did IM and Peds was beneficial. They hit the CXR hard during radiology, as well as good introductions to other modalities. That made me feel quite comfortable when put on the spot by an internist to read some patient's CXR for the team. I also think it was good to know early in my rotations just what it is you're getting when you order a film/study on a patient and how radiologists tend to think about problems.
If you were asking about electives, sorry this was irrelevant.