I agree with asking the owner, just to make sure that all bases are covered. The horse may have had a biting issue in the past, so now they are super careful about who does what, you never know. I would much rather someone pet/kiss my horse than randomly walk by always feeding them treats by hand, especially if the horse has an allergy, or starts to get nippy. That being said, I agree that I don't want my horses to always be put in a bubble, and no one can do anything with them unless I'm there, they deserve lots of attention, and in the case of my Arabian, she was soooooo one person oriented because of crap in her past that I practically threw her at other people, because I wanted her to learn that other people were safe as well. But if I happen to have a horse I know is nippy, or invades your space, I will nicely ask people to not do certain things so that everyone is safe, and I don't have to undo training, or issues caused by other people doing certain things that just don't work with said horse. Quite a few horses I know don't mind or even really like kissing and hugs, but just about fall over in happiness if you actually scratch their itchy spots instead, much like a herd mate will.