There are a few things to consider if you whiten before crowns and veneers. If porcelain will cover ALL teeth that show, there isn't much point, as the desired color can be prescribed to the lab. If some natural teeth will show, they can be whitened and the final shade of porcelain can be chosen to match. But consider that ALL bleaching is temporary. In time, teeth will darken, perhaps only slightly. If the veneers are thin, this darkening will shine through and the veneers will protect the teeth from whitening gels for future treatment. Or in the case where some teeth not covered in porcelain still show in a smile, these teeth will need to be maintained to keep them matching the adjacent teeth. If the decision is already made and whitening IS to be done, it should be done prior to porcelain. Whitening is unpredictable, so you can't "bleach to match" (place veneers on uppers and bleach lowers afterwards) as the teeth may not whiten as much as hoped. It is not unusual to bleach, place veneers, and then years later be disappointed. The smile that was perfect on the day the veneers were placed is now yellower, as the veneers were SO thin that the natural darkening of teeth is shining through (or edges of veneers show due to natural tooth showing). Dentists without enough experience would not know this, and would not know to avoid the situation.