As with everything else about pregnancy, the laying on the back thing varies from woman to woman, and from baby to baby.
And, also with many other things in pregnancy, there's a common sense answer - if it feels ok, it probably is ok. If the weight of the pregnant uterus pressing on the vena cava isn't causing oxygen deprivation to the mom, then it most likely isn't causing oxygen deprivation to the babe.
I've known women who were upset because when they woke up at night to pee they had been lying on their backs... when I would ask if they were comfortable on their backs they would say "oh, yes - but (fill in the blank) told me you weren't supposed to! But I just keep ending up on my back during the night!"
Hope this helps - personally, I've had it both ways; and of course, if I got short of breath, I would move.