I'm in zone 5, but I've already picked my butternuts, most of them anyway, using the fingernail test. Other things to look for is die-back in the vines. Their stem will be dry and hard, starting to turn brown. I try not to disconnect the stem from the butternut, it leaves a raw spot, so take clippers and cut a couple inches of stem with it.
I've never grown pumpkins, but in most of the fields I've seen they seem to let them lie a long time, the vines pretty much dead. The gourds I raise store better like that too. I leave those 'til frost. (Of course the pumkins are going to get picked for halloween, and usually by then we haven't had our first real frost here)