If you're stupid enough to bench heavy on your own, have sense enough to not put collars on the bar, at least to have a chance of deloading the weights off of one side. I personally would never recommend a smith machine. I if the weight is heavy enough to pin you it's more than likely going to be too heavy to roll the bar back on the rack or let alone push it back up to the next notch to rack it.
Some years back we had a local kid that decided to workout alone in I think it was his grandfathers garage equipped with a smith machine. The kid got pinned and was found a couple of hours later, dead.