I think it depends on the clerk - there were a couple of associates that were so stingy that if I saw they were cutting, I'd leave the store and not buy anything.
I've actually (hold your breath here) had a manager at WalMart give me a few inches extra.
And others that cut it to the nearest thread - on their side of the mark!
I've also had clerks try to stretch the fabric before cutting it.
I forgot to add to my washing post earlier - the fabric may have been cut "straight" - perpendicular to the fold and selvage - but after washing the ends may or may not be even if one folds the fabric selvage to selvage.
So many of the directions say "fold your fabric in half - -- " It's important that the fabric is not wrinkled at the fold line - forget about lining up the cut ends -when the fabric is folded, it should lay smoothly and have no wrinkles - this is where "straightening/truing the ends comes in.