I leave the foliage alone and try to obscure it with plants that develop after the daffs bloom. Just last weekend I participated in a daffodil show sponsored by the American Daffodil Society where there was a guest speaker from the UK who is considered an expert in the field. A lady in the audience asked about cutting foliage 6 weeks after bloom and he strongly recommended against cutting or diminishing its ability to replenish the bulb. He said that it will diminish the number of blooms you see the following year.
Once I start seeing yellow in the leaves I yank them off- they're pretty much done at that point. The speaker said to avoid using any kind of knife or cutter on daffs (mostly referring to the blooms) because if there is virus lurking it can be spread to other plants with tools.
