Getting rubbery is too late. Damage can begin when you get to the rubbery stage. My recommendation is to stop before rubbery. That would be as I recommend above. When you reach the point where you start loosing control... this is natures way of telling you your muscle fibers are fatiguing. Fatigue is an early warning sign. As you continue past fatigue, abnormal fiber use ensues to compensate. These fibers are not use to the stress they must endure to compensate. This is where micro tearing of fibers start. If you stop at this point, damage is reversible, but the more strain you put on them the more unnatural stretch the fibers will make. This takes them out of their coordinated fibrile bundle. If this continues, remodeling will happen. With remodeling, you get permanent change, and this changes your embouchure... often times, in a negative way.