Usually only your school cares where you do core rotations. Mine specifies that we have to do them at a core site but I think you can get special permission to do some elsewhere if you have a compelling reason. The program you visit may have restrictions based on the rotation, i.e. some specialties only take 4th year students, some will only take you from July-December if you are auditioning as part of a residency application or plan on applying to that specialty. If your school is ok with it get in touch with the programs you are interested in an ask or look at their requirements on VSAS.