I also do one every time even though I've used the same one now for ages, as you say allergy can kick in any time, and the thing with allergy to hair dye is that it's very extreme and can be life threatening. I've heard of several deaths, and have seen many articles in magazines of people with swollen heads and hair loss
No reputable hair salon will dye without doing one these days, even Supercuts did one for B when she went for colouring........I'm glad you do it Loz
I know many don't bother, but with my food allergies and the skin issues (eczema and dermatitis and now possibly psoriasis) I would be stupid not to
Let us know how you get on with the new colour Loz, ammonia free is much nicer for the condition