Most people just don't undertsand about nitrate film. I have spoken to people who literally think it will explode. In order to calm them down, I tell them how to look for the markings "Safety" or "Nitrate." I explain the difference between black letters on clear or clear letters on black. Finally, if there is any ambiguity, I tell them to try the "float test" or the "flame test."
I just had to go through this whole rigamarole two weeks ago. I guy found some old films at a garage sale and somebody warned him that "Old movies are made of nitrate film" and "Nitrate film catches fire!" It took me two days to get the guy calmed down. I had to finally meet up with him, inspect the film for him and perform a flame test in order to satisfy him that his film wasn't going to burn his house down.
You and I both know that marking it "Safety Film" is superfluous because there are no manufacturers producing nitrate anymore. However, most people react to things they don't understand with fear and panic.
My suggestion is this: If it doesn't cost anything extra and it takes no more extra time than to thread the film around one more roller, there's no reason not to do it. If you ever have a customer who doesn't want his film marked this way, it's a small matter to skip the step.