a quality carnauba paste wax (eg. ~ meguiar's) should not leave residue... liquids and inexpensive paste waxes (eg. ~ turtle wax) will leave residue.

also share with them your past/negative experiences... and you do not want them to leave any extra residue or wax in the cracks, crevices and/or trim areas.

in a lot of cases... you get what you pay for.