Above what level are gmat scores treated the same ?

There are three reasons to retake in that range.

1) You blew it on the quant section (aka Q42 V50 for a 730 or something like that) Even with a high overall score, a really low quant score can hurt your overall chances.
2) You are going for a scholarship. Often times a really high GMAT can help if you absolutely need a scholarship, or think you are strong enough in other areas to earn a merrit scholarship but need a 99% score
3) You left a bunch of points on the table. If all your practice tests are at 770 and you scored a 710, then it's worth a retake b/c 60 points more is obviously going to help your application.

With a 710-730, your GMAT won't KEEP you out of any school in the world. But at the same time a 770 will not GET you into any top school in the country. Once you've hit a strong score, it's your job to bring a strong application in every other area. At a certain point, you have to decide how your limited time and resources are best spent to improve your application. If you are shooting for a 10-30 point increase on the GMAT (which is never a guarantee) your time is most likely better spent in other areas. A higher GMAT score is always better, but you have to consider the opportunity cost of taking the GMAT again.