Your body will tell you when it's all right to sleep on whichever side, or you stomach, for that matter. There is no rule about it. Some people find they need to sleep in a lounge chair or similar for days or weeks, others sleep fine in a bed right from the start (I did). If it feels all right, go ahead. You may find that trying your left side gives you palpitations or just a really uncomfortable or painful feeling for a few weeks or months, so you might want to skip that for now. And you may not want to lie on your stomach for a while, just because it's hard to get up out of it. However, there is no rule that you have to sleep on your back after OHS, and no damage should be done if you don't.
As far as stretching your chest, you absolutely should not remain in a hunched-over position or sitting in an armchair in a way that makes your chest concave (slouching). You need to breathe fully, and slouching will prevent you from using the lower portion of the lung lobes. In some cases, the very base portion of the lung there can collapse from being forced down by the ribs and fuse together permanently. Your lungs are recovering from surgery and anesthesia nearly as much as your heart is.
You may feel that you're protecting your chest by not stretching it at all, but you are setting yourself up for possible extended, even permanent rib pain, such as costochondritis. You need to expand your rib cage, to ensure your rib cage heals in the right shape and the cartilage, tendons, and tiny muscles regain their proper relationships with each other, and to ensure that you're breathing properly. Practice breathing with your entire lung capacity as many times a day as you can remember to do it (meaning slowly breathe in ALL the way).
Your chest is wired together with stainless steel. What you are avoiding are violent or weight-bearing movements, such as opening/closing a car door, picking up a gallon of milk (a pint is a pound - it's over your limit), handling anything over your head (even be careful shampooing), or thoughtlessly pushing yourself up from a prone or seated position. Simple motions won't cause any problems.
To loosen up your healing chest muscles and tendons, you do need some movement. Begin stretching slowly and carefully. Don't stretch past your limits, but do stretch your limits just a little. Don't use rapid or vigorous movements, as they may be difficult to control at this point. Don't use any weights of any kind. As things loosens up, try gingerly twisting at the waist from side to side. Again, slow and gentle are the watchwords. Try doing these things briefly, but at numerous times during the day, such as every time you get up. As the great Satchel Paige once advised, "Sort of jangle around when you walk, to keep yourself loose."
Nothing can guarantee you won't ever have chest pain later, but some activities like this can help avoid unnecessary pain from shortened tendons and intercostals (tiny muscles that run between the ribs).
Sit up and stand up straight, shoulders up and back, just like your mother told you. You should be able to do this even right after surgery. You breathe better and you heal straighter. Make it the way you want it to be when it heals solid.
Best wishes,
