I'm a little bit obsessed with accents, perhaps in part because 85% of the time I meet someone, I have some version of this exchange:
"Where are you from?"
"What happened to your accent?"
[Explanation that variously includes me pointing out that regional accents tend to be muted in large metro areas, that I've moved around a lot, that I work with people from all over the world, and that I'm married to a Canadian. Some of which may or may not even be explanatory, in fact.]
In historical movies about the American Revolutionary period, or in documentaries where actors read the words of various historical people "in character", Thomas Jefferson is frequently given the genteel, upper crust southern accent most people would associate with a wealthy Virginia farmer. George Washington, on the other hand--who fits the exact same demographic--almost never is. And I cannot think of an example in which John Adams or John Hancock are made to sound like they are from Boston.
Of course, we have no idea what any of these men really sounded like. In fact, we can really only make an educated guess about what anyone sounded like in 18th century America. But I'd be willing to bet that we would have a hard time recognizing their regional accents today. After all, if you listen to how people spoke in movies from the early 20th century--or even better, in "man on the street" type interviews from the early days of radio, it's pretty clear that Americans now don't even really sound like they did 70 years ago, much less 240.
Given the years he spent in the British military, I'd be willing to bet Washington sounded a lot more English than Jefferson did. But then again, there's really no reason to assume that English accents in the 18th century sound like they do now, either.