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I definitely notice when a pretty girl has her hair fixed up, her neck can be exquisite.
Things that make a girl's neck attractive are the taper from her shoulders to her ears, whether its a gradual rounded and smooth taper not clunky. A noticeable v shape in front can be a turn on as well.
Small peach fuzz under on the back of the neck can be sexy.
So yeah some of us notice. I think it might have some level of primitiveness to it as well, like it shows submission and vulnerability if a girl has her hair up exposing her most vital weakness, her neck.