We don't give headings other than 0 or 5 because it's less difficult to have a misunderstanding with only two possibilities.
At my centre we see the downlinked Mode S heading so we know what it should be after an instruction to turn 10 degrees.
This also means that anyone rounding up or down gets "caught". Rounding bugs me. If I wanted the aircraft on a particular heading I would have said. If I want the aircraft to turn 5 degrees left then I want it 5 degrees left.