An original quality screen will always sit perfectly flush in an unbent frame even without any adhesive.
The key to a professional quality mini screen replacement is:
1.) Removing every last trace of the old adhesive
2.) Making the frame perfectly straight/corners perfect (use a g-tool or a dremel)
3.) Making sure the digitizer and lcd flex are folded as in the original---many people allow the digitizer flex to stick to the glass, which will cause it to bend the wrong way and cause a crease and a lift on your screen.
The other common cause of a lift is any bend whatsoever in the large metal shield that the LCD sits on. If yours is bent, it would be better to just leave it off entirely.