I don't know about the OP, but I certainly wasn't thinking about "whipping out the Pert and lathering up" nor am I worried about "shiny and luxurious" hair when SHTF. (though wow, Pert, do they still make that stuff? and suddenly I'm reminded of Flex too, with the cowboyhat and all.
LOL ) I'm assuming you're just joking in a general way, because otherwise that's actually pretty insulting to anyone who has posted suggestions here.
Hygiene is a fundamental part of good health, as well as helping with morale. I've gone all kinds of grubby when camping, but I also still took care of basic hygiene: tooth brushing, face washing, daily wipe-downs or sponge baths, weekly shower and/or scalp washing. I've learned how to do that without much water, or to bring some extra water if I wanted to spluge.
Also, keep in mind that it's much easier for most men (or women with short cuts) to go for weeks without washing their hair as they usually have very very short hair or virtually no hair. It doesn't come into play for them so it's easy for them to dismiss the issue. Women (and men) who have longer hair, especially shoulder length or longer, need some kind of maintenance. Not for prettiness, but just for basic sanity. Long hair gets matted, knotted, tangled, itchy, it's in your face, it's distracting, dirt and dust stick in it, it caught in things. 15 minutes of care goes a long way to avoiding all that.