The tape will help with the sandblasting effect, as Chris points out. A bra is essential if you value the paint on your front hood, fender and valence.
Duct tape is a catch-all label that can mean a lot of different things. Some products with that name will use adhesives that will either lift your paint off, or leave behind gummy residue. Racers' tape (or gaffer's tape, a product that's scarce outside of L.A. or New York) both have much kinder adhesives.
Here's what unprotected D.E.ing leads to on a front valence:
