Whether you do testing beforehand or not is entirely your own decision and two fingers to anyone who demands otherwise
The contention comes along when no testing has taken place,problems arise,which they can,and the installer then poses questions on a forum for faults that may have become apparent had prior tests been made,that action angers many
I can honestly say I never test anything before arriving on the job and giving a price.if problems arise.I sort them as part of the job
On the other hand,virtually all of my work is for people who are customers and friends or long term select builders,so trust is not in question
If you are doing work for customers who may be unaware of necessary work that may accompany a cons unit change,then it becomes very important to do some basic tests,both for you and your customers benefit
Surprises,both in work required,then costs incurred.it can be a very big bone of contention