The reason why you should not wash your hair is because the natural oils on your scalp protect the scalp from chemical products put on the scalp by the stylist. That is the short version.
However, if the hair is very dirty, or oily the hair will need to be treated prior to chemical service given.
Here is another however, your stylist does not want you to wash your hair because you, the client usually will scrub the scalp and irritate it. (you know scratch it so good). This in turn makes the scalp irritated and most likely to get burned.
So now you know DO NOT WASH YOUR OWN HAIR 24 HOURS PRIOR TO CHEMICAL SERVICE!
One the rare occasions when a faithful client washes hair prior to service, (shhh don't tell anyone) if we have known them for a long time and have a great working relationship with them...we do the service.( Let me add there are different precautions we take tweeking chemicals so they are not as harsh). Haircolor 102.
However with on the scalp bleach washing prior to service is a NO NO!! If your stylist wants to bleach out your scalp and you have washed your hair before your appointment, you better run, or reschedule your appointment.. You can take that to the bank! So now you know and are informed from a real stylist to you!