I should take some data. I've got enough cats to border crazy-cat-lady territory. I had the impression that they sleep in random positions -- basically, whatever shape they were in when the urge to nap came upon them.
I'll keep an eye on them and see if they have a preference that I'd missed. It won't be all that useful -- if nothing else, the specific preferred sleeping places of my house could have more to do with my layout than any underlying mechanism in the cat. But I'd kinda like to know if there has been something staring me in the face all this time and I just didn't put it together.
I resemble this remark. My wife and I run a feral/stray cat rescue, and anecdotally, this seems to be true, with a few caveats. In particular, the ferals tend to sleep with their backs to a wall, which overrides the left/right preference, the strays not so much.