I agree 100% with Paul. Using a polarized plug on a layout assures that all transformers are phased, even when too dark to see a red dot, or when someone else is running and doesn't think about the red dot.
It is a safety feature, because having transformers out of phase can cause some dangerous shorts & current flows on the low voltage side. 40 volts can give a tingle if your hands are damp, like from sweat.
Having said that, let me add that I repair transformers for MY layout. If I were doing them for resale, I'd use a non-polarized plug unless a buyer specifically asked to match his/her existing transformer.
If the layout is fed from multiple receptacles, then one must check to determine if the receptacles are on opposite sides of the 220-volt line.
Tripleo: household receptacles are never wired in series. They are wired in parallel. Use of white wire, with no marks, is a sign that your house was wired by an amateur. Be careful! The lack of a ground wire also entails some risk.
SW Hiawatha. The postwar ZW can be wired either way. Unless someone has tinkered inside or a wire has broken and is touching, the two wires are interchangeable, and there is nothing for a third grounding wire to connect to. I I use 2-prong polarized plugs on my transformers so that they are always in phase. My entire layout is fed from one 20-amp receptacle, which I know is wired correctly because I wired my house.
One can never assume that because a house was wired by a professional or was passed by local inspectors, that it is correct. One of my sons bought a new house and I found the GFI in the kitchen was wired backwards. Another bought an old house. When we renovated the kitchen, I found the 20-amp outlets were fed from each end by different breakers.