I watch them lay out the fabric and then make them measure from the shortest part. They get annoyed with me too. I tell them that I want a yard, a full 36". Not 36" at one end and 32" at the other.
I had a cutter tell me they couldn't do it and I asked them politely to get a manager. The piece in question was horribly cut previously and if I hadn't made a fuss the cutter was going to give me a really weird shaped piece. The manager came and told her to do what I said.