Just, for comparison, look at what they call common, like seawater in a port city (it's in the air anywhere near the Sound) or sunlight. Then you decide if cats and dogs cut it as "common."
Really though, it all depends on your game. If every (80% or more?) private residence you enter is going to have cat hair, then it's common. So, it would be a likely penalty every time that character entered someone's home, apartment, billet or whatever, (which shouldn't be too often in most games, but can be.) Basically, if you want it to be fair, work it out between the GM and player. Discuss how common "common" is from a defined mechanical standpoint and play with the interpretation you're both happy with. But, hey, if you as the GM want to give a player a little extra BP on the free, there's really nothing stopping you. You don't need to try and game the system to fit into a 400BP mold. Make characters you're happy with, then play!
Personally, I don't see wage slaves being allowed living pets (I imagine most people who can afford to spend the majority of their free time in Utopian VR worlds instead of the sickly, cramped world they live in,) and I don't see barrens folk letting a good meal sit on their lap instead of in their stomach. But, like I said, every 6th world is different.