When the trade is between coverage and orbital altitude, coverage wins and closer is not necessarily better. It is also more dangerous to insert a spacecraft into such a close orbit, where it is also affected more by the perturbations induced by the body it is orbiting and thus more difficult to navigate. Look into the history of the MESSENGER mission and you'll see that once complete coverage of the surface was obtained the orbit was lowered progressively until impact was achieved at end of life, this seems to be the scenario you are asking for.