You should try to avoid doing anything that will irritate your skin immediately before treatment. This includes sunbathing (natural or artificial). Practitioners are unlikely to treat someone who has sun burn for example, as they would be more likely to suffer pain from the addition of heat to the already damaged skin; however an established tan would be fine.
Some women find that their skin is more sensitive right before and during their menstrual cycle so if you are a woman, you might want to take this effect on pain threshold into consideration when scheduling your treatment appointment(s).