I would myself always regard it as incorrect:
1. Mr & Mrs Johnson =>
2. The Johnsons =>
3. The Johnsons' house.
4. Don't we have to go round the Johnsons' tonight?
5. Did you hear? The Johnsons' Siamese cat has just been stolen by a gang of international pet thieves.
(That would be how you'd find it in a newspaper or book published by a larger publishing house, for instance.)
All the best,