To avoid scratches from wayward putty knife behaviour, masking tape along the thin perimeter edge of the aluminum case can be your friend. :-)
Alternatively, Good/High quality, *Clear* packing tape (parcel tape) is friendly, too. In my experience, the better quality tapes don't leave a residue when you peel it off of the aluminum case. For easier application to the thin lip of the case, cut a length of tape, then split it down the middle.
My anecdotal tips:
1. Working with old or dried-out masking tape is a maddening exercise. If the tape tears or breaks as you peel it off the roll, it's time for a new roll.
2. If, when you hear the word "tape", your knee-jerk reaction is to grab "duct tape", then Stop! Put down the duct tape. Take a deep breath and walk away from the roll. Using Jack Bauer measures to leverage your Mac Mini with duct tape a putty knife is sure to leave a sticky mess after you've negotiated, or in Jack's case, "softened-up" the housing.
Chloe O'Brian: Jack, um, I know it's not your style -- please don't use the duct tape.