Hi Martymm

***Finding the support stiffness and entering a conservative value into CII is quite simple so why not.***
If you find support stiffness yes you can use it for advantage & it is realistic also. My opinion is if you want to use support stiffness use it for all the supports in the calculation.
Do you take help from structural guys to find the support stiffness values?
If so it is not much cost effective to get so. More over you need to judge how much practical advantages you are getting for the same rather than using support rigid stiffness ( bit more conservative)
Alternatively you may go for composite analysis( piping + structural model in Caesar).
For some special case finding support stiffness & take it's advantage to pass sensitive equipment nozzle load may be considered.

Regards

Habib