I realize it may be better to reopen the unit and replace the screws, but I still have the first iPad 2 I replaced the screen on and I completely misplaced my LCD screws. I just decided to put pressure on the back of the unit to hold the adhesive to both the covered glass and unit.
I've found over the last few years in the dusty environment of my farming community that it works just great anywhere. However I can understand that you may not want the LCD placing any pressure on the backside of the new digitizer.
If you are proficient at getting back in the unit I'd try to put them where they belong but mine works just great the way it is without the corner support screws.