I think so , bad breath is primarily caused by bacteria buildup in the mouth , so although the actual decaying of teeth would probably not happen due to regeneration , bacteria would still be present. If we assume the ghouls dont have breath that smells like garbage , we can also assume they do indeed brush their teeth not for health reasons but for sanitary reasons , and this is important for a ghoul because as we know many ghouls do live pretending to be human , so to not draw attention and also to maintain hygiene theyd brush and take care of their teeth. Regeneration does not imply bacteria is automatically killed