to be honest I would definitely charge something for this reason:
I am starting my owm mobile business very soon. when I was training I refused to accept money even when it was offered, as I was only a trainee, didnt feel I could justify charging, and because I explained to the "client" that I would do free nails in exchange for their honest opinion. that worked fine whilst training, but now I'm going professional, you wouldnt believe the trouble I'm having trying to get it through to some people that they have to pay now, and that payment cannot be in the form of the odd bottle of wine!! I've been very direct and to the point, but people can be very cheeky, and will still assume that you'll do it free just for them even when you've told them you wont! Fair enough I have one friend who still gets freebies in exchange for her being my guinea pig to try new stuff on, but you cant give freebies to everyone or whats the point in going into business??
Charge something, you'll b glad you did when it comes to being in business.