Caution
I'd like to argue a little bit.
Experience tells me that certain notices and disclaimers only come about because some court somewhere in an applicable jurisdiction has declared that because, in one instance, said notice or disclaimer was not posted that a breach did not occur, a promise was not broken, or that in general there is not grounds for recovery for the party exhibiting a little bit of sense.
So then, I suggest that unless a person (real or imaginary) does not specify that cancellations must be made 24 hours in advance/cancellations will be grounds for recovery efforts/shoplifters will be prosecuted, etc., etc., that plaintiff/defendant does not have a completely solid legal and metaphorical leg to stand on. I'm not saying that they definitely will not be successful in litigation, and I'm not saying that I agree with the rationale. What I am saying is that at some point Vidal Sassoon or Revlon or Redken or Conair was successfully sued because someone was injured or killed while "improperly" using a hairdryer manufactured by Vidal Sassoon/Revlon/Redken/Conair. And now there is a new notice on the tag on the cord that explicitly states, "Caution: Do not use while sleeping".
People (attorneys) are getting much more literal, much pickier, and much more ridiculous with the argument of "But, I didn't know, they didn't tell me!", and so I argue that the "service" "begins" at the time the customer arrives for the appointment and "work" is "actually" "started". Absent some sort "of" notification about a cancellation/promptness policy, a customer who simply does not show up may not be in breach if the deciding court is one of the insane ones.