I have always used Iron sights (when I was in Reserves), everyone should practise reflective shooting...in Airsoft most contacts are 30 metres minus (which in most cases where the Assault begins), you should be able to line your sights on any tgt in that range and hit it.
As far as the SAW goes, I usually watch where the "Beaton Zone" or spray of the BB's go. When I taught on the Machinegunners course (years ago) one thing I use to preach is to watch where the strike, tracers and Beaton Zone fall, that is where the cone of fire is, that is where it is effective. Thats why sometimes having a an optical sight on a machine gun is useless. Yeah it is effective to ly up with your tgt but a Machine gun is more effective if you can hit that tgt with a cone of fire (grazing fire) of the gun, watch where the strike and tracers go creating that cone of fire (beaton zone).