This thread, and some subsequent Googling, is making me feel so much better! It seems like there really is no consensus and lots of people are just as confused as I am.
There seems to be an even split between people who think that laser is a medical service and that you wouldn't tip the tech anymore than you would tip a dental hygienest or sonogram tech, and people who believe that laser is a cosmetic service like a facial and that the person should be tipped just like any aesthetician.
I can see both sides of that argument, especially since the prices of laser treatments can vary so much. I was paying over $360 for 30 minute sessions at my dermatologist and I never even thought about tipping on that. Now that I'm at a medi-spa instead of a doctor's office and I'm paying $38 for 15 minute sessions, tipping seems more appropriate.
That being said, I did think it was a bit cheeky for the receptionist to ask for a tip so boldly. I felt really put on the spot--there was no way to decline without looking bad (which wound up happening anyway) which sort of takes away the "optional" part of a gratuity. They could have just had a discreet sign or put out some envelopes and a drop box like they do at salons.
After reading the helpful responses here and doing more research, I think I'm going to tip $15/session, including the one I missed. There doesn't seem to be a straight answer on what the "real" price is for laser, so I'm figuring that between my 15 minutes of treatment time and another 15 minutes for setup and cleanup, I'm taking 30 minutes of the woman's time. Tipping $15 for that works out to $30/hour or roughly $60,000/year. That seems like a fair wage for running a wand over my arms. I don't think I'm going to be going back after I complete this session of treatments, though. Having the tip issue pushed in my face so directly and being embarrassed by that left a bad taste in my mouth.