I have a feeling that it is more to do with historical reasons. When hand flying an aircraft it is (was) easier to fly headings in multiples of five. Nothing to do with piloting skills, but more to do with how the compass is marked (the 5th and 10th degree tended to have a thicker marking than the individual degrees). Especially relevant if flying on standby instruments.
Having said that, in the military where (I assume) there is more practice and currency in manual flying than on civvy street, then the use of intermediate headings, especially on a PAR, was given. Even then though, for changes of one degree it used to be the norm that you would be told to turn left 3 degrees, then turn right 2 degrees, to get a one degree left adjustment.
Re above regarding reciprocal headings for separation... if you're worried that a one or two degree anomaly is going to give you convergence on aircraft 6 miles apart head to head, then you are cutting it a bit fine with your separation anyways and I'd suggest that on radar no one is that good at judging distances...
Nowadays, you can tell a pilot to fly a heading of 141 degrees and he turns the knob to that heading...
Last edited by anotherthing; 14th Sep 2013 at 07:19.