It's all relative. In animal terms, say compared with a Komodo Dragon which has enough bacteria in its mouth to down an elephant, then yes, a dog has a relatively clean mouth. That doesn't mean you should practice your kissing techniques on Rover - dogs' mouths still carry all manner of bacteria which is useful to them, but can give humans a nasty turn. If in doubt, just watch the places your dog puts its mouth in an average day - you'll soon get the idea!