A question about sitting, movement and mindfulness

I have had bad posture for most of my life. I find that if I do not sit quite straight during sitting meditation, my body leans forward putting weight on my legs which becomes quite painful. However, when I sit straight, my "sitting bones" become painful. So, I tend to rock from side to side while sitting. It may or may not be visible to others, but I am aware of it. Perhaps "shifting my weight" might be a better way to describe it. My question is regarding mindfulness meditation. Do you think it is OK to rock side to side? Should I just note something like "rocking, rocking"? Should I wait until the pain is nearly "unbearable", and note "pain, pain", "disliking, disliking" before shifting my weight, or is it OK to do so pre-emptively?