With a well trained, respectful gelding I'm a little more relaxed than I am with a stallion. Stallion, mouth my direction even with no contact, correction. Gelding depends on the personality. I had a QH that LOVED to lip me. He did what I call 'the cookie duster' with his upper lip and would swish it back and forth on some part of me when I'd be grooming on his favorite spots. That's all he ever offered to do, so I never corrected him and we were fine for 30 years. He's also the only horse I've ever allowed to rub his face on me, he understood the boundaries. After a ride, after the bridle was off and the halter was around his neck, I'd brace myself for a good rub on my back. He'd rub his itchy face and crack my back after a good long ride, at the same time. We both loved it. I have NOT found that level of trust with any other horse, so beware.
I think 98% of the time, the mantra of, "A lip leads to a bite" to be good advice and I'm also wary of hand treats, though I do feed them to horses that demonstrate that they understand the etiquette. Anyone who gets too carried away and demanding.....cut off. I think generalizations with horses and people can be a little too cut and dried and you need to take your horses actions and personality into account.