bob
It's better to have two hands than one glove.
I'm curious - Do you give your customer everything they paid for?
They paid for a sign and that's what they get. How they get it is my concern, not theirs. The vast majority of waste goes to the dump. Life is too short to be constantly wrangling odd sized pieces of material.
Meaning, if you charge them for 11' of 36" banner material do you hand that over with their 1x10 banner? That's like having a substrate cut down and your supplier not giving you the drop but being charged for the entire sheet. Also why would your customer have to pay for 36" material instead of 24" or 30" which is readily available? They shouldn't have to pay because you don't stock the right size.
I just don't see how that is right. I'm not harping on your process I just don't agree with it. If it works for you then have at it.
Your concept of right and my own seem to vary considerably. Actually, my lack of interest in what might be right for anyone other than myself approaches total.
I stock what I use 90% of the time. The other 10% just has to fit in somehow.
Moreover, except for certain exotic substrates, the incremental cost to the client is minuscule relative to the total cost of the job, as is most all material cost, whether it's used or tossed on the burn pile. They pay for what I unroll and/or whatever I cut out of. If someone finds that unreasonable, they're welcome to hie themselves to some other shop.
Perhaps after decades of schlepping the same bunch of scraps back and forth around the shop you might come to feel pretty much the same way.