I probably wouldn't retake. 720 is high enough that your application is unlikely to get cut from consideration based only on GMAT. In addition, if a school is going to make you an offer with say a 740 GMAT they are probably also going to offer you at a 720 GMAT. Plus, there is always the possibility you do not improve, or maybe even score a bit worse the second time around. Once you're above the schools self-defined GMAT threshold then the marginal benefit of a few more points on the GMAT is very low, in my opinion.