It's best to worm only if you think there's a problem. The over use of wormers has lead to resistance and it is especially bad in animals such as sheep and cattle due to its over use on farms, rendering worming treatments a lot less effective (in some cases completely ineffective) for those animals.
Using deworming medication doesn't ever wipe out all of the parasites. Some have a natural immunity so if you are frequently using deworming treatments you are increasing the likelihood of resistant worms breeding with each other, creating more resistant worms, until they make up the majority of the population and your treatments no longer work.
You can get a fecal float test done through a vet to see if there is a large enough population of parasites in your flock to require treatment but most healthy birds can cope with a certain worm burden with their immune systems keeping the population in check.
Changing which brand/type of wormer you use when you do treat reduces the likelihood of a build up of resistance because different individual parasites are resistant to different types of medication. I'm in a different country so I can't advise you about specific ones, but I've probably only treated ours for worms twice in the 7 years we've been keeping birds and I've never had an issue.