NOT a fan of smith squats...
1. The machine locks the body into the machine's groove, and prevents the body from determining its groove.
2. If you stand with your feet pretty much underneath the bar, the knee flexion will be exaggerated, and stress there exaggerated too.
3. If you put your feet forward, to reduce knee flexion, the lower back is put in a weak position, with rounding of the lower back likely. In addition, there is additional stress on the knees as the feet want to slide forward but don't because of the friction from the floor surface.
All in all, the Smith machine squat may give an illusion of safety relative to the barbell squat, but in reality it's a bad exercise. Sure there are degrees of "bad" depending on the specific form used, and the individual, but it's still a bad exercise. Rather than try to minimize the problems with a bad exercise, why not just stick to exercises that have far better potential? Of course, even the "safe" alternatives can be ruinous if not done properly, but best to stick with movements that have good potential.