a disc can lose stability through wear or through rim bending. The wear factor is not a consistent factor and depends on the original shape of the disc. A frisbee shaped disc will be much more affected by wear on the bottom edge than a Roc, but a Roc will be affected over time too.
Bending or tacoing a disc will not necessarily change the stability either. Discs with wide rims are more vulnerable to having their long noses bent, and it is this, rather than the over all bent or straight shape of the entire disc that causes turn over.
By far, the most important part of any shot is what is happening in the last split second as the disc is pulling itself from your grip. Focus there. It's the key.