I often do not patch test for allergic reactions. It's bad, I know! I want to do this for box dyes but I don't know HOW am I supposed to patch test these products.
For those that have separate bottle for the peroxide and a tube of dye, you can put a little of both in a separate container, mix them, then test on yourself. Unfortunately, the boxes I've bought have a non-sealable container for the peroxide. Then I think to myself: this stuff will decompose pretty quickly and I need to keep this on for 48 hours (H2O2 -> H2O). So I end up putting it on anyway without a patch test :( I take it all off if it itches, but it's not a good way to do it.
I do patch test for facial products (because it's really easy!). However, recently, I patch tested a product on my jaw line for 7 hours. It was fine. However, when I used it the next day, it seems I had an allergic reaction after 5 hours on my face! It was horrible :| I'm still paying for it right now.
Urgh. I don't know how to explain it. Maybe my face is just weird.