As an employer (in the US) I have always had the impression that I'd risk being on the hook for overtime or some other form of additional compensation if I routinely engaged in communication with employees out of hours, so unless the place was on fire I never did. As an "employee" (contractor) I managed my own hours and adopted a pragmatic approach: if the client was paying $$$ I'd respond if it didn't interfere with my personal life, unless they were being dickish about it, in which case I'd bring it up as an issue and if necessary drop the client.