I don't know about you guys but here in Maryland the vets recommend getting them checked for worms twice a year. And only worming them if they have a parasite. Because the horses are becoming immune to the wormers because of over use.
But I understand if your horses do have worms then worming would be a problem. :)
We use different wormers everytime so that the horses don't come immune to them. And also apple flavoured wormers don't work for tess, she isnt stupid lol she knows what it is and she don't want anything to do with it'!