From what I understand of the thread so far, I think there's been some misunderstanding going on - am I right in thinking mrsmop that your children have either self-taught themselves or have had to sleep on their sides for different reasons - a couple because of reflux, one for breathing problems etc?
I think with all the literature and talks that new parents are given over the risks of cot death, one would have to be a very blasé parent to escape the information that babies are safest lying on their backs. But the OP's actual question was about the safety of allowing them to sleep on their side. I've had both back sleepers, side sleepers and front sleepers. Some have co-slept, all have eventually gone into a cot. Mrsmop is right to say that you can't be there all night every night to see if your baby stays on it's back, and yes, it is up to you as to whether you want to disturb your baby's sleep to get them back on to their back, or to allow them to sleep undisturbed on their side. However we ALL agree that, theoretically, in a perfect world, we would have our babies' on their back all the time. But there are those of us (strangely enough, it seems to be the ones who've had more children than the norm of 1 or 2), who've had experience where to leave the child to sleep peacefully is a more practical way of dealing with life with a small child.
As always, no one is wrong, no one is right, but OP, you must do what you feel is right for you and DS. With my first, I tried (and failed) to get her to sleep on her back, ending up with co-sleeping with her to ensure this happened, meaning I got no sleep because I was perpetually paranoid that she would asphixiate. With DC4, I co-slept again, but bolstered him either side to prevent rollovers. Once he got to rolling over by himself though, there was no stopping him. Now this is a baby who doesn't like being on his back for a nappy change, let alone for a whole night's sleep. Now he's 19 months, sometimes in the bed, sometimes out, but he'll sleep in ALL sorts of positions, even face down, bum in the air!
Of course you should know the facts and figures, as with any major decision in parenting, but you need to make an informed decision as to what is more important - an undisturbed sleep (and subsequent routine that will likely come of it) and to take the risk of sleeping on his side, or to reduce that risk totally but drive yourself barmy with lack of sleep? :o