Lying on your left is OPTIMUM - obviously you're going to move in your sleep, well, unless you're too big to move like me, but it's still a completely true, sensible and important point.
The main issue, though, is that you will know if you are doing yourself a mischief because you will start to feel pretty ropey (as outlined by Apostrophe)... and that is your body telling you to move.
With DS1 I was happy to carry on sleeping on my left as I had always done, but this time I found it relaly difficult to do for some reason; until just a few weeks ago. For the last 5 weeks, if I lie on anything other than my right side I feel sick, dizzy and scarily faint for someone who was lying down.. I think this baby is definitely taking up more space than the first.
"During your first trimester, get into the habit of sleeping on your left side. Sleeping on this side benefits your baby by maximising the flow of blood and nutrients to the placenta. It also helps your kidneys to expel waste products and fluids from your body more efficiently, which in turn reduces swelling in your ankles, feet, and hands." from babycenter.com.au
www.birth.com.au/Belly-piercing-and-tattoos/About-lying-on-your-back.aspx
www.babycenter.com/404_whats-the-best-sleep-position-during-pregnancy_7608.bc