Here's my take on it. If your customers specify the hardness, case depth, etc. in every case I would say you can opt out. If there are some cases where the customer says "here's what I want to do with this part" and you specify how you meet the requirement, then you are designing part of the product. If you assist them in coming up with appropriate heat treating, and they put it on their drawing, specification, or PO, then they are still responsible for the design.
Of course if you want to do it anyway (to improve your business, not just meet the requirement), you can always include 7.3 because a note in the standard says that it may also be applied to the development of product realization processes.