Personally I would not lie about my EDD.

At my first appointment the MW put be a few days further back, (based on one cycle that went a couple fo days longer than normal. But when I had the dating scan, it went back to the date based on my LMP.

Having had a prem first time round and having a 50% chance it will happen again there would be no way I would consider lying about dates. Due to fast pregressing labour (7-8cm by the time I got to hosy), it was too late for steroids for bubs to develop her lungs prior to delivery. If it happens again and we can get there quick enough to stop labour progressing I would be much happier for bubs to have received the steroids than not IYKWIM.

It may also change the course of care for bubs when delivered.