[broadgage]

I suspect that often effects do cancel out, but in some circumstances even a single load drawing several amps of DC current from AC mains can be significant.
A relatively long AC main and the transformer winding supplying it might have a resistance of 0.1 ohm, which at say 250 amps load might equate to 250 volts at the transformer and 225 volts at the far end.
A load drawing 4 amps DC near the far end of this main would result in about 400mv of DC voltage drop which as noted in the first post can be significant.

Hairdryers are normally only used briefly but a failed high wattage discharge lamp might draw several amps a DC for week or more.