Thanks for the suggestion MotorDavid, I've been burned in the past with things like that so I checked for correct mounting before I left when the tires were originally installed.

Ard, I think you may be on to something, 500 miles is about the right distance to scrub off any mfg residue, release agents etc so perhaps the tires now have enough grip to force the toe in/toe out oscillation you suggest. The shop had a quick look last night and did not see any uneven wear patterns or tire defects so I've scheduled an alignment adjustment Sat morning. Keeping my fingers crossed.