I don't think there's any universal or easy solution. For one, each situation is different. Some dogs are harmless, but others can and will bite, especially when you trigger their predator instinct by "fleeing".

Carrying pepper spray can't hurt, but it shouldn't be the only or first option. Like others have said, the wind might be blowing the wrong direction, or you might fumble it in the heat of the chase, or you might not be able to effectively deploy it while at the same time handling your bike and negotiating other obstacles. If I carried pepper spray, I would only do so as a back up to the protocol, if you will, that I've developed:

It involves making some split second decisions and then acting on them immediately.

1) Can I out run the dog? If I'm traveling fast and can easily accelerate and the dog didn't get the jump on me, then I just pedal faster.

2) Is the dog aggressive or is it just playful? The breed is a good indication -- lab vs pitbull for instance -- but mainly it's a question of whether the animal seems happy or pissed. It isn't hard to distinguish.

3) If the answer to the first question is no, and the dog indeed seems aggressive, then I slam on the brakes, get off the bike and confront the darn thing, even sometimes charge back at it. I've never had a dog not back off at that point. You have to show them you're not afraid.

If at this point you still didn't feel safe and in control, then it would certainly be appropriate to use pepper spray.

It's never "okay" that dogs run out onto a public road and chase bicyclists, but they are predators by instinct, and bicycles can be quite rare in many rural areas, so quite a novelty for the animal. It's simply going to happen. IMHO, Pepper spray is a rather extreme measure, and vengeful if it's not absolutely necessary. I can understand why an owner might get pretty upset seeing his or her animal doused and wrything in pain unless you've exhausted all your options and it is still clearly menacing you.

It's still quite a drag to have your peace of mind (not to mention your forward progress) so viciously interrupted, and it can leave you rather angry because it seems that there's something personal in the attack, as if the dog is a proxy or extension of the ignorant redneck owner. But in reality, the animal is just responding to a rather unusual occurance -- a bicycle going by -- and most owners are quite apologetic. I've started to see it as just an inevitability of bike touring and a small adversity to be dealt with just like a flat tire or a bridge out. Calmly take care of the situation and move on.