Teeth Whitening Before Dental Crowns and Veneers?

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.

Absolutely whiten your smile before you begin any dental treatment!  Whitening is not forever but you can whiten more than once in your lifetime.  It is exciting to have a new smile with crowns and veneers but doing whitening first will clean up your smile and give you an idea of how white you really can go.You never know-if the whitening is effective sometimes you don't need to do as much work on your smile.  There are all kinds of whitening options so do your research!  Much success.

If I understand you correctly, you will be covering the top 8 teeth in your mouth.  Sometimes more than this number of teeth show when you smile.  If you are planning to make your crowns and veneers whiter than your natural teeth that show, now would be the best time to bleach your teeth.  You could then pick a lighter shade for your restorations that matches or blends with the lighter shade of your teeth.  It also depends if your bottom teeth show when you smile.  Some people only show their upper teeth and then it is not as crucial to whiten the bottom ones.  Some people, like movie star Julia Roberts for example, show upper and lower teeth when they smile and for people like that it would be good to whiten their teeth first.

It all depends on your GOALS for your mouth.  Look at your smile see what makes you most comfortable.