The external lookout was an integral part of my training from the very first lesson and the school were very keen on it. I had five instructors over my PPL course (our instructors were very popular with the airlines) and all insisted on good lookout and telling them of any traffic I saw. Only one of these was unhappy about me looking behind and above on short final, telling me "not to worry, there's no b**** Meschersmitts behind you!" (I'd added this scan as a C152 had caught us up and tried to land on us just before the threshold on a non-radio airfield when I'd been with my previous instructor, getting to within a hundred feet of us before he realised that we were there.)
My last instructor was very keen on using the landing light at all times when the visibility was poor, or we were operating low down in areas where Shawbury's helicopters went to play, or when there was a low sun behind us. He reasoned that just because we were looking, it didn't mean that we'd always spot something else and it also made us much more visible to head-on traffic.
Since then I've always done this and made a point of briefing my passengers to report all sightings of other aicraft.