OK, let me try, but also state that this is purely my own opinion, as seen from Scandinavia/Northern Europe!
First, if starting with 'the helicopter view' both piping and integrated valves are always part of a larger system, including other machinery and perhaps several kind of processes integrated in a 'plant' or by building a 'project'.
The project could be for petrochemical purposes offshore or onshore, water purification, distribution or cleaning, chemical processes, gas, steam or a large number of other things, and located anywhere in the world.
What would then decide the piping specification and valve specification, and how independent are the one of the other?
Again from the top down:
a) Any local rules or special rules related to the location and process. General area (US, BS, EN, JIS etc), auditing companys (UL, DNV, etc.. ) process locally (radiation, emission, explosive, sour service etc). Will apply to the whole project.
b) Any specific demands set down by the end user. will apply to the whole project.
c ) 'Normal best practice' related to the process and point a).. (how we usually do things here...), but also commercially related to best/cheapest available comercially at the given location. Will apply to the whole project.
d) The valves and the piping must obviously fit together and are as such not unindepent of each other. The valve specification should however not be decided by the piping engineer alone, but be cross-corelated to the overall project specifications as indicated above.
e) At an early project description stage it would always be wise to 'open up' for alternative bidding for any mechanical device, as long as point a) and b) are satisfied.
I do not know if theese roundabout comments will help you. Correlation between piping and valve specification are newly discussed, try to search all forums.
Good luck!