I usually have no problem with dogs in town, but further I go outside into the villages, more often some dogs start to chase me. Usually dismounting is enough, but it depends on situation. We had some unpleasant encounters in Romania, but there you can find big packs of wild dogs. You can read more under Animals here: http://www.romuni.eu/post/55772254187/summary-part-2-last
You can have also some more reading on dogs on this forums:
Dog Bite
UnPlanned Dog Dismounts
Dogs... is it just me?
Some people wrote that ultrasonic dog repellers work. I was using just a whistle, which was not enough for some dogs and barely better than correctly timed shout.