Not a mistress here, but have some input. The defense mechanism of stinging nettles works through little microscopic hairs that are only about a millimeter long or less. They slightly penetrate the skin and break open releasing several bioactive compounds that essentially just function as irritants or irritant enhancers. The primary irritant is formic acid, which gives it that stinging sensation. Other compounds are things like histamine and acetylcholine. It will definitely cause redness and rash in most people and is often said it can last up to 24 hours. In healthy individuals, the irritant is just that, something that causes pain and discomfort, but usually nothing the body won't recover from.
The hesitancy I would have is, as with any organic component in play, anyone, healthy or not, can develop allergies to things out of the blue. That's especially true for organics that get closer and closer to the blood stream and for organics that are already trying to negatively affect you with compounds the body doesn't expect. Some people have developed allergies to stinging nettles. In CBT, it probably has a higher chance of triggering allergic response due to thinner, more sensitive skin, the extra blood flow, and more vasculature.
All that is not to say "don't do it!" just arming you with information to make your own decision and so you can discuss with potential subs.
Interestingly, the capsaicin brought up before, which is the active ingredient in hot chili peppers, affected me in an unexpected way a few years ago. I've eaten extremely spicy food my whole life and cut and processed peppers many times by hand with no issues. A few years ago I bought some jalapeños and sliced them up (while wearing gloves), and my hands swelled up to about twice their size, became really itchy and extremely painful. For probably a day I couldn't really bend my fingers and they still had some issues the next day. Never had that happen before that, and no longer cut up my own peppers.
