Craig at GeneaBlogie poses an interesting question: Why do we so often slip into a hillbilly accent when pretending to talk like old folks—even if our folks grew up in Manhattan or Saskatchewan? The answer he provides cites Looney Tunes, so it must be correct.
Actually I alternate. When I am doing my poor imitation of my Pennsylvania ancestors, I use a dadgum sort of accent, all "well, hail Columbia, young'uns!" And when I do the tiresome impression of the NYC ancestors, I sound like Aunt Tessie from Greenpernt. At least I'M having fun!
I almost lost my Maine Yankee accent when in college, but it comes back whenever I spend time with my father. I can't really call it an "impression" of my ancestors, since—somewhere deep down inside—it's still my natural speaking voice.