I know when I did hockey jerseys I was using the full width of whatever roll I had, and more length. Can’t remember what width it was, but think it was wider than 15. But I’ll order whatever I need for most projects. And I’m still not checked out on the DMS vinyl cutter.
But, for Tshirt uses, I could see value in being able to do 15x24, or 5x7. Scrap is likely going to be a challenge either way, but purely from an advise perspective, I would probably target maximum usability.