What an interesting thread?
Very different in the UK to here in Australia.
Although I'd recommend a patch test for those who have never had a spray tan before or known allergies, the onus, at the end of the day is on the manufacturer of the tanning solution.
Therefore, if a client does react, the manufacturers insurance covers any possible claim.
It is important to know what ingredients are in the solution you are using. All tanning companies should clearly detail the ingredients in their solution. These should also clearly be detailed on sample sized bottles as well.
Wondering - does everyone patch test their client when they do a facial?