I have been testing and designing electronic fuel injectors for more than two decades and have yet to figure out why Bosch and some others indicate polarity. There is nothing but a coil inside conventional electromagnetic injectors. I have done extensive magnetic testing and there is a small difference the shape of the first pulse when you reverse polarity due to hysterisis. Any trace of this is gone by the 3rd pulse so it is insignificant.

There is a significant back EMF when the magnetic field collapses but this is absorbed by either an RC or (more commonly) zener circuit in the ECU. The breakdown voltage of the insulation is typically more than an order of magnitude higher than the back EMF voltage.

Siemens, Delphi, ourselves and many others do not indicate polarity.