There has been a lot of discussion on this issue; you may want to search old threads... The fact of a matter is there is no simple answer. In my experience, both fellow applicants as well as clinical advisors seem to be evenly divided on this issue. If you decide to do an elective you better make sure you can shine, i.e. know basic eye anatomy, physiology, pathology, basic exam, ddx, etc.,etc. No one will ever be impressed with a medical student standing in the corner and simply observing. Remember, in a competative field like ophtho it easier to exclude rather than include people. You also run a risk of just "getting on someone's bad side", i.e. someone on the ad com may dislike you for whatever reason; even if you otherwise do an excellent job. On the other hand, if you are interested in a particular program (especially if it is not top tier), doing a rotation may be enough boost for a successful match.
Case in point: if you decide to rotate, make sure you know your stuff.