In a nutshell . . .
This is how we do it:
Top to bottom, front to back and from wingtip to wing root.
Top to bottom: requires less deicing fluids and gives a better view on the to be treated surfaces such as the vertical stabilizer (less steam to hinder the view when starting from top to bottom). Also reduces the chance of flooding the APU inlet. On the fuselage, deicing fluid is sprayed above the cabin window line which will then drip downward. This prevents spraying directly on to the windows which should be avoided, same goes for the cockpit windscreen and windows.
Top to bottom: also prevents spraying onto pitot tubes, static ports or ram air scoops on the lower half of the fuselage which should be avoided for obvious reasons.
Front to back: to have a good overview on the wing (and stabilizer) leading edges and forward, most critical area of the wing. Also better to manage application of deicing fluids using the leading edges as a reference. A good application technique is to cover the leading edge and upper surface of the wing with a minimum amount of fluids dripping on the tarmac from leading- and trailing edges. Spraying fluids from aft to front over the wing does not give an acceptable view on the leading edges and a lot more fluid will drip from the trailing edge than necessary. Also spraying from aft to front over the wing risks more fluid entering the "quiet areas" such as speed brake/lift dumper cavities and trailing edges with cable runs. Residual deicing fluids in "quiet areas", turn into a gell over time which may negatively affect flight controls. Also a higher risk of damaging static discharger wicks on trailing edge surfaces when spraying from the back to front (spray nozzle closer to the dischargers).
From wingtip to wing root: gives a better overview manouevring toward and around engine nacelles and toward the fuselage. Due to wing dihedral on most aircraft types, less fluid is required because there is better control over the fluid path flowing toward the lowest point, the wing root, which is always in view spraying from tip to root.
Furthermore, on remote spots, we direct aircraft position depending on wind direction. When ever possible we put the aircraft with the nose into the wind so any spray will go from front to back.
Regards,
Green-dot