Are you considering naturalizing them? Daffodils described as good naturalizers won't have any problem with the grass, and will persist as long as the foliage matures before mowing. I have several naturalized in thick pasture grasses, where the hay isn't cut till after the Daff leaves are dry. I've found early varieties are best, as the foliage on those ripens earlier and I don't have to wait so long to mow. If it's a short variety, like Tete a tete, you may even be able to mow on a high setting without removing much foliage.
