Short answer - No, never.
Long answer – You should never feed ducks bread, whether they are wild or domestic. It is very damaging to a flock, and potentially devastating to an overall ecosystem.
Here is the explanation of how feeding ducks bread is damaging, for both wild and domestic flocks
Wild – Wild ducks are not pet ducks. They do not need to have human interference with their food sources. Furthermore, they can easily become reliant on humans tossing them bread, and neglect their foraging skills to naturally acquire food. This has to potential to knock an entire ecosystem out of balance, as the food chain will become altered by the introduction of a new food source.
Domestic – Obviously, your pet mallards are already reliant on you for their food. While they may have foraging skills, they already depend on you for their sustenance. Is bread okay then? No, not at all. The reasoning here is that bread is not an adequate source of nutrition for ducks. It does not contain the protein levels and nutritional diversity that waterfowl need to thrive.
Furthermore, ducks have a trait which causes them to stop eating once they are full, they won’t overeat, regardless of how much actual nutrition they’ve acquired (perhaps it would be good if some humans developed this). Thus, they’ll fill themselves up on bread, and ignore the other food available to them once they’re full. This will yield an undernourished flock.
Now most of us grew up feeding ducks and other birds bread when we were kids. It was an engaging and simple way to interact with wildlife. However, as our conscious evolves to better understand our impact on the environment, it has become painfully apparant just how improper it is to feed wildlife. We must ignore some of the fond memories that we’ve had with this practice, and learn to love and enjoy waterfowl and other animals through observation, not interaction.