I tend to give such situations the benefit of the doubt and:spam: assume they are training their dog to not run up without permission but aren't quite there yet in which case the last thing I want to do is reinforce that nonsense and increase the likelihood it will happen again.
And I for one will talk to people while my dog will stand there looking at the person and in my case not saying no is not implied permission and kneeling down or going to pet my dog without me releasing them first puts me in an awkward position.
I guess I don't see why it's hard to just...ask. Or ignore the dog. Or ignore a moment until you can ask. Obviously if the dog is all up in your grill that is one thing and you may have to react but that doesn't mean you have to go reinforcing that by petting and talking them up without a quick check with their person first.
Thanks for the interesting response; hopefully I can clarify my original post. We might still have differing opinions at the end of my post, but I hope that I can at least give a few examples to help make transparent my thoughts. When I made my initial comment regarding conversation etiquette, I was referring to situations in which the owner allows the dog to move forward, not in which the third party moves toward the dog. For example, I would never reach down to a dog that is in a sit-stay beside its owner without asking first. Nor would I approach a dog that is tugging at the end of its leash if it required me to "make the first move" so to speak. Let me give just a bit of reference to a situation that I experienced that fits into this category. I had a scenario in NYC this Feb when I was speaking to a woman who was going to be running her dog in the agility portion of Westminster. We were both in the hotel sitting about 5 or 6 feet apart in the lounge chairs. Her dog was on a leash, but she was not holding on to it, so the dog was not actually restrained, although it was very well behaved. The dog walked over to me and put its head in my lap as we were conversing. The owner made no comment or move to bring the dog back to her nor did I react negatively to being approached by the dog, and so I took that as unspoken understanding that I was accepting of being approached by the dog and that she was accepting of the dog being near me.
As a minor tangent, one could argue that just because the owner was accepting of the dog approaching me did not mean that she wished for me to touch the dog; however, I tend to think that my response (that is, petting the dog briefly), was so typical in such a situation that she would have inferred that it was a potential consequence of letting her dog visit me. If she had wished differently, she should have said something (e.g. "he's quite shy, I hope you don't mind if I let him approach, but could you not reach out to him") or restrained the dog. Had I jumped up and down squealing, hit the dog with a frying pan, or squeezed its jowls and stuck my face right up in its grill - no, that would not have been appropriate EVEN IF the owner allowed the dog to enter my space. None of those actions ought to be an "expected" consequence of letting a dog approach a person - simple petting? Personally, I think that it is.
Back to the original situation - the owner was looking at me and so should have had no doubt as to where her dog was located. Should I have asked? Maybe. Could I have asked? Definitely. Would it have been hard for me to ask? No. But it just didn't feel necessary (or even the socially appropriate thing to do) in that situation. So I gave the dog a scratch behind the ears and continued to talk with the woman.
I still stand by my opinions on "if a dog runs up to me off leash." I don't think that anything is black-and-white, so I'm sure there are situations in which I would not feel comfortable touching a friendly dog that had run up to me (off leash) uninvited, but generally, yes, I would. Provided that the dog was friendly and I was in a position to be approached (i.e. not walking another DA dog, etc.), I would never be anything but polite and smiley with the owner, but the fact of the matter is that the dog entered into my space. If the owner was actively calling it back, I would not "reward the dog" by petting it, but typically in my experience, these situations have happened when the owner was either not present, not paying attention, or not giving a crap.
The "on-leash" but not under control is a bit more variable from situation to situation, but I try to assess every situation socially to determine how to act. Don't get me wrong - I will typically ask before petting, but I have been in situations when it didn't seem the appropriate thing to do. No owner has ever made any negative comment to me, so I'd like to think I usually make fairly good judgment calls.
I should mention that my interaction with dogs is much more limited to the general dog owning population than many on this forum. I don't do dog sports, I have been to few dog events (although I would love to change this!), and I don't have too many "dog people" as friends. More often than not, my dog interactions are with owners who probably haven't given much of a second thought to the matter of "petting with or without permission."
As a side question for discussion, what are everyone's personal opinions on petting another person's dog when you are invited into their house? That is, they invite you over for dinner or something of the sort and they have a dog. Do you always ask before petting then? Or do you not? Or does it depend?
Personally, I see so many situational factors that it is hard for me to make a blanket statement.