This is to the guys who want to wash a girl's hair: GO FOR IT!
Really, what are you afraid of? Washing a girl's hair is one of the sexiest, most sensual things you can do for her. I know you guys are shy about it, because actually having a fetish for it may seem weird at first - hey, if they don't do it in mainstream porn, it's not considered normal - but I'll bet anything most women would be all for it if you brought it up. So take a deep breath, and say "hey, I'd really like to wash your hair."
I think the biggest reason a woman might hesitate is that she's afraid you'll mess up her hair or damage it. Assure her you know what you're doing and you'll be careful. Also assure her that you'll shampoo her in a position that will allow her to stay fully clothed, in case that's a concern as well. Finally, assure her that you expect nothing in return - that should always be the case, but make sure that's always known.
Most women love having their hair washed! Why else would a shampoo be part of every haircut? Why else would Herbal Essences think to make those commercials?
If you're really in doubt and want to ease into bringing up the idea of a shampoo, start by offering a shoulder massage. If she seems to enjoy that, move your hands up to her scalp and see how she reacts. If she enjoys that as well, then offer to wash her hair. Make it about her, frame it as a favor for her, not something you're doing for your own pleasure (you may be perceived as a creep otherwise). After you wash her hair, then you can say that you enjoyed it - she'll love that you enjoy doing something so pleasurable for her.