You dont get any distortion on the actual model as far as i'm aware there stretched as the UV's change even though its laid out quite well it will never be 100%
What I like to do is to not UV at all, sculpt the model than take the levle 1 out and UV that and swap it over using a morph, that was any small changes in hte UV/vertex postition that may occour when sculpting are taken into acount, this may be your problem if if its a heavily sculpted mesh
"No pressure, no diamonds" Thomas Carlyle