In your case I think the adcom will question your motive. You already have 2 extremely good scores that can get you into any business school, but you seem obsessed with getting a higher score.
I don't think that a higher score will work to your advantage anyway. I think that you should now concentrate on your essays etc.

I agree. You took it twice, once when you felt poorly prepared, and again when you felt well prepared. Your scores were within 20 points of one another, which means the difference is statistically insignificant.

At this point, you might be able to take it again and get 20-30 points higher. Or, you could get another 680, or perhaps 670. Why take that chance?

The adcom isn't really going to care that much about whether you got a 700 or a 730. However, they are going to look down on you taking it again if you get a lower score on your third try. They appreciate persistence, but not ad absurdum. They are also looking for people with common sense, who know when to say good enough.

A 700 is not a deal breaker for your application to any school, even the top 10.

Here is straight from Harvard's website about the GMAT:

The Admissions Board hopes that all applicants will recognize the following: the full range of GMAT scores is very broad, that there is no "minimum" score requirement, and that the GMAT is just one piece of data among the many used to evaluate an application. 50% of the Class of 2009 scored between 700 and 740 on the GMAT.

You will notice that your score is mentioned as being in the range of 50% of the class. Basically, as much as we all like to obsess on the GMAT, you are at the point where Harvard doesn't really care about your GMAT. It is going to be the other stuff they focus on.