Umm, i would have thought you will always get rounding converting between the two, in blitz3d you need to force a quart to euler conversion to see the error:
if you stopped editing your posts it would be interesting to compare the errors
Graphics3D 640,480 c=CreatePivot() TurnEntity c,0,30,0 TurnEntity c,0,-30,0 Print EntityYaw(c)
if you stopped editing your posts it would be interesting to compare the errors