With my DS, I had a scan at 5 weeks 4 days and they used that as my EDD. At my 12 week scan I was measuring 4 days ahead, but the GP and the hospital made me keep my earliest scan date as my EDD, hence why I'm conflicted!!!
Totally understand. With the twins I had no idea of my dates, so just went off 1st dating scan which was close to my LMP date. At the 12wk scan I was 5 days ahead and remained that way in subsequent scans but they still went off 1st scan..

There can be a couple of factors influencing what you would prefer. I would prefer the hospital thinks I am further along because IF i did go early, it could be the difference between Bub being legally recognised as a birth at 20wks, or being legally recognised as viable at 24wks.. (Commonly people don't worry about this though, I think it's just because of my history) Then there's the other side of the coin as Kass mentions, if they think you are earlier, they will let you go longer before talk of intervention.. At the end of the day though EDD's don't mean much at all!