I do believe I've heard that excessive stretching is a sign of hip dysplasia. As far as I know, THAT is the myth; HD is very real. I don't recall where I heard the excessive stretching part. I agree with everyone else though. If you are unsure (or just curious) get some xrays and send them to OFA. My dog's hips are rated OFA Good and she stretches regularly and often.
C-ATCH Pimg - DOB: 2/06, RATO, UJJ (x2), DJA, HIT, CA, CGC, High in Games, Reserve High in Standard- CPE Nationals 2014
RATCHX2 High Jinks vom Neuanfang - DOB 9/12 (Gotchya Day: 1/23/2013) DN, CA, agility superstar in training