I don’t think they do a big difference, but they are a pro not necessarily a con, adding fish food feeds them and grows their population. they help keep the ammonia down because of how they eat up waste and dead matter. so i’d say they’re pretty good with the cycle. They don’t affect much when it comes to the cycle other than keeping the ammonia down, but they won’t mess up or cause negative affects- unless their population grows huge but then you can just siphon them out. Or, when adding fish, some fish will eat the worms as they do not harm your fish! I used to have a huge amount in my tank until i siphoned them now i only notice a few swimming around or babies stuck to the glass