Tipping for Laser Hair Removal

[ChiKate]

If you've had Laser Hair Removal, do you tip every time you go? Do you tip a percentage or a set amount?

I'm currently going to a med spa, which I read you aren't required to tip at, but there's little envelopes on the front desk so I'm assuming that implies you should tip. I did not tip last time but was only there 15 mins for underarm laser and bought an additional package to return 6 more times. I'm going tomorrow for bikini which I know takes longer so I'm wondering if I should start tipping now? Or maybe in the middle of the sessions? What is the proper etiquette? TIA!

[lilygirl]

I thought you were asking about tipping at London Heathrow.

No advice but I'm interested too as I've just bought a package of 6 today.