The seeing eye dog etiquette?

There was a beautiful seeing eye dog in training with a sighted trainer on the bus today. It was all I could do to resist trying to make friends with him/her(the dog,not the trainer).

Is it poor form or frowned upon to pet a seeing eye dog when it isn't actively guiding an unsighted person?
Update: The handler was deep in conversation with her friend,it would have been ill mannered of me to interrupt.

I'm glad I restrained myself.
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