Speaking strictly in my own opinion:
Tips are appreciated but never expected and I really think it never crosses the mind of most people whether they 'should' tip or not. I do think (remember, this is my own opinion) that if a wedding party books with us and we're killing ourselves trying to do 8 people plus the bride on a busy saturday, it would be nice to throw the artist a tip. We know darn well that you tipped the hairdresser for the updo that you hate, so why wouldnt you tip for a beautiful makeup job?
It's just a nice gesture and it makes me feel appreciated knowing I made you feel gorgeous!
either way, if you love something an artist did ...why not tip? or I like the idea of a coffee too lol. A tip is a reward for a job well done isnt it? How many times have you come home with a shitty haircut or color , dropped a bundle of $$ for it and tipped the person who ruined your hair/life?? lol !!!