yes, the last thing that you want when doing a leg press if for your lower back to round and/or come off the bench, this will cause lower back injury. (trust me I have done it on more than one occasion!)
You should never go below the point when your butt lifts off the seat....thus causing your back to round. Keep doing what you have been doing...there is no need to go any deeper than 90 degrees. Past this point not only stresses the lower back but it also puts added tension on the knees.