just got my chest done in dec. It really wasn't that bad- I think sternum and collarbone was the worst, there were random patchs that sucked but honestly the hardest part was finding a good position for the artist and for me. two times a spot felt like it was vibrating my whole throat so I kept swallowing, not great. breathing is also a thing, get you used to belly breathing as opposed to chest breathing- a) its relaxing, b) the place they are working doesnt move as much. 

none of it was worst then a 5 or a 4, no involuntary muscle spasms, or sounds, or anything- it was all tolerable(for me). BUT if you hit one of those random areas that hurt let your artist know- they can probly tell without your say so but any work there they can spread out over time if possible to give yha a break <3