Tipping at medical spas?

I get massater Botox for TMJ and it usually comes out to 650 dollars because I need a lot for how powerful my jaw is. My place has never asked for a tip at checkout, but when I went yesterday it asked if I wanted to tip 20%, 18%, or 15%. That is an 130 dollar tip. Thank god I pressed skip, because I felt an immense amount of guilt after. I googled it, and it said that medical professionals (people who handle needles) should NEVER accept a tip. I always tip at massages, nails, facials, etc. 20% at least! For those who get Botox, do you tip?