Whenever I feed fruit or vegetables, I have fruit fly maggots in the bin. Does this cause a problem for the worms?
No, all part of the process. The issue for most people is the fruit flies themselves if the bin is inside the house.
Freezing fruit (then thawing) before adding can help if you want to cut down the number of flies. Also covering feedings up as well as smaller feedings that get eaten more quickly.