If the fabric is is patterned you have to cut with the pattern, but I agree with WendyW that that can make the curtains hank wonkily.
Ideally look at the fabric on the bolt. If the end is cut with the printed pattern assess how far from true that cut end is, or if it's not cut with the pattern fold it back so that it is and then assess. If it's more than about an inch out across the width reject it and choose something else.
I know that curtain fabric can be horribly expensive, but I suspect that, in general, the more expensive fabrics will be better printed than budget fabrics.