Another fed.gov employee here.
I'll answer emails on my time off, but if it's more than about 1 sentance, that answer is "I'll take care of this tomorrow/monday" as far as actual work, they pay me overtime or wait till my work hours. I'm not interested in comp time, as I already have use or lose vacation. There was one slightly awkward conversation when I told my boss's boss "No, not doing comp time. Either it's important enough to the command to pay me overtime, or it's unimportant enough to wait till Monday. " I got overtime.
I think as soon as you hit management level anywhere you can't completely check out. Subordinates need to be able to get ahold of you, and Seniors will have the occasional question outside business hours. It's important not to let yourself be abused, but we don't need laws for that.