I think that's very belts and braces. But potentially a disclaimer could be worded to show it has been explained to the client that a patch test could cause an allergic reaction, and accordingly they gave accepted that risk as part of the procedure. This would certainly have some merit in defending a negligence claim.
However, I think the the extent to which patch tests in general are useful is open to debate since allergies can be developed over time, and even if testing negative to a patch can still go on to have an allergy during treatment. I personally don't patch (but my insurance/training provider do not require it) and I use a patented ultra sensitive adhesive. I'm comfortable with this decision as clients have a full indepth consultation to rule out any potential medical issue that might give rise to an allergy, and sign a full and thorough disclaimer confirming they have had the risks of a reaction fully explained to them.
My advice though is always to follow your manuf/training provider recommendations otherwise your likely to breach the terms of your insurance, and be found liable for negligence if sued.