It depends on how much you trust your candling and what you hope to gain by taking them out. I don’t hatch ducks, but they take longer than chickens. I’m not sure at what rate they develop and what you should be seeing.
I do know that with my colored and darker chicken eggs it can be hard to see inside at all at any stage. Even when I can see inside, I hardly ever take them out at 7 days when I first candle unless there is a very clear blood ring. I just don’t see a need to. I just mark them with an x, check, or question mark. I do take out the obviously bad ones when I go into lockdown.
The danger is that an egg could explode or leak a real stinky mess if bacteria get inside and multiply. Whether it is fertile or not has nothing to do with that. I always clean my incubator well between hatches, only set clean eggs, and keep my hands fairly clean and dry when I handle the eggs, so it’s not been a big problem. If you’ve ever smelled a rotten egg you understand how bad it could be so I don’t take it real lightly. I rely on sniffing the eggs when I add water or candle.
If you trust your candling, there is no real reason to leave them in. On the other hand, there is very little risk in leaving them in. I don’t see this as having a right answer or a wrong answer, but pretty much personal preference. But if you smell anything off, sniff each egg individually to check them out. You can usually identify the culprit pretty easily.