IMHO, I think it would be best to bring it up casually first, just to see how she feels concerning the subject. A conversation that wouldn't be sudden and catch her uncomfortable or unexpected. A small comment to feel her out on the subject; for example, compliment her hair in it's current state, to show her that it isn't something that goes unnoticed. Try to bring it up casually and then go from there if she seems to be open to the subject. If you are together I would assume that she would like to make you as happy as possible while she remains in her comfort zone. It is her body after all, and you don't want to make her feel like your violating that by being to stern or demanding. Just my thoughts, best of luck!