Is Zoom whitening necessary for my sensitive already a1 shade teeth?

Your dentist recommended Zoom because the new veneer need a shade/color. If you are happy with your current shade on the natural teeth, then tell him or her that. If you feel you want to lighten your natural teeth and match those with new veneers, then do the whitening/zoom. The veneers will not change color but your natural teeth will only get darker...bleach trays for 'touch-ups' are also useful when you have veneers, next to natural teeth. A1 is light but B1 is the lightest shade...probably too white. Have an in-depth conversation with the dentist about this. I would recommend doing the bleaching before the veneers but if you do not see yourself maintaining it with trays at home and you are happy with your current shade, then skip it.

If you are comfortable with the A1 shade I would forgo the bleaching and match the new veneers to the A1 shade