Answer

In Jewish law, the rule is that anything forbidden in public is also forbidden in private. But regarding matters outside Jewish law, there is no such rule, and therefore even if there is some value in manners (which I am not sure about), that does not make it binding in private. There is, of course, value in restraint and self-control, but not in specific manners as such (for example, holding the knife in the right hand and the like).