Greg,
well done for pointing this out, so many people do not think about the fluid and the changes in it's make up that occur when you are flowing injectors straight from an engine, the fuel will contaminate the testing fluid, by how much and how soon depends on the amount of use the system gets and it's especially important if you are carrying out OE comparison testing.
Of course, in the factory the injectors are new and their is no contamination of the test fluid, but where the testing guys like yourself are doing, it's very difficult to ensure all the gasoline has been removed from the injectors unless you ultrasonically clean them prior to testing, which would be my recommendation.
As you rightly pointed out, Heptane is expensive, so using the ASNU Flow Rite is a cheaper option, but as you have said, experience now shows you that you will now need to clean the system and replace the fluid and ensure all the injectors are as good ASNU before testing them.
For those doing non oe comparison testing, like normal injector testing and servicing, the fluid is ok as your main aim is to ensure all the injectors are performing the same.
A very valid comment and one that everyone carrying out oe comparison testing or characterisation of injectors must take into consideration and ensure they maintain correct fluid specifications.
Phil.