Final answer:
The incorrect waxing guideline is checking the temperature of the wax behind the client's ear; rather, it should be checked on the inside of the wrist or with a professional thermometer.
Step-by-step explanation:
The guideline that is not true for all waxing procedures is "check the temperature of the wax behind the client's ear." This is not a common practice nor is it recommended due to the sensitivity of the area and the potential for harm. Instead, the temperature of the wax should be checked on the inside of your wrist or using a professional wax thermometer. The other options, including never double-dipping the spatula, checking the wax temperature on the inside of your wrist, and advising clients to avoid the sun immediately following a waxing service, are all correct guidelines and should be followed to ensure a safe and effective waxing session.