I don't tip every time but if the service is great, and they bring a smile to my face I always thank them and leave a tip. This goes for any service I receive. I got my nails done last week and the young girl that did them was so friendly and sweet. It really was a total pleasure getting them done. I was relaxed and thoroughly enjoyed myself so I thanked her and gave her a $10 tip when she was done.
It must be hard to keep a smile on your face all the time and always be happy and welcoming. And some wait staff really go the extra mile. I think its nice to sometimes leave a tip to show you appreciate the effort. And yes I think it's pretty standard that most people leave a tip IF the service was exceptional.