Stretching your "back"? You want to have stability in this area not mobility (no spine stretches, but lats and traps are ok).
Smith machine is not an option. Foot placement relative to the bar either increases the shear forces at the knee as the foot goes further forward (the position that would have least back shear), or 1000x shear at the hip as the feet get closer to under the bar (hip will translate to the low back). Either option is cutting off your nose to spite your face.
I would prefer to see 1 legged exercises (split squats, lunges, step-ups, etc) and front/back squats done with free weights. In this manner as long as good form is maintained then relative stressors will be minimised while lifting.