I had an issue with this same thing very recently!
Had a lady cutting my fabric with scissors and trying to only give me exactly on the line...except she was not holding the fabric still while cutting!
I had to ask her to measure the end opposite to her, and sure enough it was too narrow.
I kept watching with every cut and would even place my hand on the corner in front of me, to help her keep it lined up accurately.
Needless to say, it was a nerve wracking experience from start to finish!!!
If your using a rotary cutter and ruler...ok, give me EXACTLY what I asked for.
If your using scissors, I think since none of us is perfect, add a scrawny 1/4" to be fair to the customer!
Who wants to have to hunt down more fabric bc they are short by a fraction of an inch...that they paid for.