The general rule for tipping is tip for things you wouldn't want to make yourself. If you don't want to make the latte yourself, then you should leave a tip. Baristas are skilled professionals. Making a latte with latte art is something that requires, talent, care, and skill. It's not the same as making a big Mac. At the coffeeshop I work at we use Square and I would say that 99%+ people tip, but every once in a while someone comes in that doesn't. Don't be that guy. To not tip after we work hard to make your latte is one of the worst insults you can give someone. It is essentially saying "your work does not matter". Also, due to the low cost of lattes (they are a fraction of the cost of a restaurant meal for example), it is generally a good idea to tip a little more percentage-wise than you would at a restaurant. If you pay $6 for a latte, it is generally good practice to tip a full dollar in addition to this.