I think that generally, it is rude to assume your dog is welcome. If your dog is a good dog, he/she might actually BE welcome, but also, still might not. I wouldn't want someone bringing their dog to my house unless I knew their dog really well, since I have birds (duh) who might prove attractive to a dog. My dad brings his dog everywhere, and sometimes it's really annoying, even though his dog is amazingly well behaved, it makes me nervous, the way she stares at my birds (she's a Border Collie), and obviously wants to chase them... even though she doesn't do it. If, however, you live on a suburban or urban block, and the yard sale is right next to the road, I don't think it's a big deal. But if you're actually going more than 10 feet onto someone's land, I think you should leave the dog at home.