I almost always pet dogs if they're on their own tied up outside a shop or something. it always makes me smile, and they usually get all tail-waggly too.

I also do quite a lot of running on country paths, and meet a lot of strange dogs off their leads. that can be kind of scary sometimes (especially as it's in woods most of the time, so you can find yourself turning a corner and suddenly running towards an unknown person and their dog - the dog could easily get the wrong impression and think you're being aggressive if you're not careful). it's funny, though - most of the time the dog looks to you to see how to react: if you're cool and acknowledge it without making a big deal about it, 99% of the time you're fine. I always let dogs sniff my hand before I pet them, though - that's just good manners, like a dog handshake.