"If it is common to tip, what is a general guideline for how much to tip them?"
Some patients tip and some do not. I believe that if anyone is handling products that you consume, such as food or cannabis, it is a good idea to tip if you plan on using them again.
Sometimes there are delivery fees and some companies pay their drivers well, but some drivers are making minimum wage and they can use the tips for gas money!
Tips are never required but always appreciated 
BartSmart
