Okay, first of all, THANK YOU for wanting to tip the shampoo girl! I have been an on-off shampoo girl at 2 salons for about 10 years now, and being 6 feet tall it is sometimes backbreaking work, especially if I'm working with dark, all-over hair color and long hair. I can spend as much as 5 minutes bent over a bowl shampooing, scrubbing, trying not to sweat on my charge...
Not every hairdresser tips the shampoo girl out either; I work with some women that will tip me out at the end of the day, and other hairdressers give me nothing no matter how many colors and perms I finish for them.
In regards to tipping, it's really a personal choice. Honestly, I'm delighted when a client approaches me and says "Thanks, you did a great job," and proceeds to hand me a tip. It tells me that I did a nice job and they appreciated it. As for tipping your hairdresser, some people like to hand them the money and others are okay with leaving it with me (I work the desk as well). I always give them the choice, as I can understand if someone's not comfortable leaving a $20 bill with a person they hardly know. My salon offers little envelopes you can write a note on and leave it for your hairdresser, those work very nicely. If you don't want to do the envelope or leave the money with the receptionist, I think it's acceptable to quietly approach your hairdresser and lay the tip on her station and say "Thanks." Trust me. They'll never be upset with you for tipping them.
And kudos to you for doing locks of love! I always get a kick out of women/girls that do that, it's always so fun to watch.