I don't know. Some people think they're fine with something at first, but when it actually happens they realize they're not so comfortable with it after all. This can definitely happen with tickling. It's rare, but I've had a few initially enthusiastic models who went as far as coming to my apartment for a shoot only to back out at the last second because they got cold feet when the situation got more real (fortunately, most models who back out of the project do so before coming for a shoot). If a girl is drunk/high she might not realize her discomfort with the situation until she sobers up/comes down, at which point it's too late. She might even do things she normally wouldn't, and she might regret the experience.
I assume drunk tickling can indeed be fun under the right circumstances, but I'd rather not take any chances with it. I don't know if it's unethical per se, but it can be kind of iffy.