The left side thing is about the baby putting pressure on your Vena Cava. But if you have this issue, you'll feel it. I occasionally start to get a bit dizzy/nauseous in certain positions on my back.
I have a snoogle pillow and use it to generally stay on either side, primarily my left but I switch to my right occasionally. If I'm on my back I'll use it to tilt one hip up a bit. BUT, I am a stomach sleeper, and with the snoogle I've devised a way I can lay on my stomach with a big hole in the middle and all the pressure on my shoulders and hips. I shape it sort of like a prenatal massage table. Its perfect for falling back to sleep in the middle of the night, which is a big issue for me.