While a 620 is a good score, it is very average when compared to application pools to top schools as you imply. The key to getting in the top schools is standing out, and a good GMAT can certainly help. just as important, however is your application package and how unique you will compare to other applicants with similar backgrounds. Short answer: if you can better your score by 50-80 points, I would retake.

Thanks mate for the reply.
I do understand what you want to say. I completely agree with you when you say that incase i am confident to increase the score by 50-80 only then i should retake. What I am trying to do is take a chance and try to increase the score FOR SCHOLARSHIP. I feel that a better GMAT Score can fetch you a scholarship. . I am not confident on a 620 for a scholarship. I think that a something more that 700 can surely have better chances of a scholarship.

though i have started working on other parts of my application, I am thinking to give another shot and take a chance.