Veneers are a synthetic material, or man-made. Similar to that of a crown or composite filling. If you tried to bleach natural teeth adjacent to artificial ones, only the natural teeth would whiten. I would presume that the same would occur with veneers, and the whitening material would have no effect. If you plan on whitening, I would suggest that you have your veneers made a few shades whiter than the teeth adjacent to them, and then match the natural teeth with a whitening agent later.