The general rule for tipping is tip for things you wouldn't want to make yourself. If you don't want to make the taco yourself, then you should leave a tip. Taco makers are skilled professionals. Making a taco with just the right amount of ingredients and getting it to fit in the wrap is something that requires, talent, care, and skill. It's not the same as making a latte where you are just pouring various liquids into a cup. To not tip after we work hard to make your taco 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 tacos, it is generally a good idea to tip a little more percentage-wise than you would at a coffee shop. If you pay $6 for a taco, it is generally good practice to tip a full dollar in addition to this.