Any void in the crown or veneer margin can harbor odor-producing bacteria. The same holds true for veneers or crowns that do not fit the tooth perfectly and are overbuilt. Food particles become trapped beneath the gum tissue and break down into plaque that contains bacteria, as well.
In addition to talking to your dentist about fit concerns, you'll want to practice proper oral care in order to keep bad breath at bay. When flossing veneers, make sure you go on top of the veneer, where it meets the tooth structure.
Additionally, veneers should be brushed with toothpastes that dissolve the protein pellicle chemically rather than through abrasion.