^ We highly suggest you do NOT use only tape.
Any cosmetic piece that is mounted against gravity (aka, upside down underneath the car should not use just tape). Tape weakens with age, heat, moisture and you don't want the piece flying off!
For the diffuser, the screws are there to give the piece a secure fit. The tape is optional but we rarely if ever use the tape during installation.
Hope this helps! You can PM me with any questions.