I got my dating scan (I knew my dates but wanted to see bub) at 6w5d. There was a visable heartbeat and bubs was measuring on my dates. Everyone will want to go with LMP, and seeing as I don't have a 28 day cycle and o'ed on cd 17, I just changed my LMP date to match my dates, which were calculated from conception, and with both the scans I've had have proven to be correct, spot on. So it's up to you, really.