And yes, you should never pet a guide dog while it is working. If you think it might possibly be an appropriate time, then ask the owner first. (Though I've never even asked unless it was someone I knew, so maybe that is also not a good idea unless you already know the owner.)
Being an animal lover, it's my natural response to want to pet a dog, cat, Iguana, whatever. It was years ago, and I was clueless on the subject at the time. I petted the dog, and the guy got pretty snippy about it. I've since read up on it, and found out why.
I just found this on Google:
How should I behave around a Guide Dog?
It takes a lot of concentration for a Guide Dog to work safely with its user. To help this team focus on their important work, please follow these tips.
* The Guide Dog must not be the centre of attention - please don't pat, feed or otherwise distract the dog when it's working. A well-intentioned pat can undo months of training.
* Please don't grab the person or the dog's harness. Ask if they need assistance first.
* When you provide guiding assistance, please walk on the person's opposite side to the Guide Dog.
* Please make sure your pet dog is on the leash or under control around the Guide Dog. When approaching, it may be polite to let the person know that you a have dog.
* If you see a loose dog, please contact the local Council.
* According to government legislation, you must allow the Guide Dog to access all places used by the public.