What to give? There are only three effective dewormers out there, Pyrantel (and it's not great), Ivermectin, and Moxidectin, and the last two are the same "class" of drugs. Praziquantel is paired with Ivermectin or Moxidectin to get rid of tape worms. Giving Pyrantel first then one of the "ectins" with Praziquantel covers everything. Easier, IMO than sending out pooh for a fecal and waiting for it to come back to you... My vet charges more than the online labs for testing fecals, and his turn-around time isn't much better, lol. For me, it's cheaper and quicker to just treat than send off fecals on all 8 horses...
If you board, then either all horses need to be on the same schedule, or all horses have to be tested and treated regularly as needed. I'd rather trust treating my own horses regularly to make sure the other boarded horses aren't contributing to my horse's worm load, than trust other owners to test/treat their horses on a regular basis... If you're going to test/treat, then you need to have a "closed herd" with just your horses, or pay to do them all yourself.
Sorry, I have to disagree. This mentality of it being "easier" is exactly how we have gotten into issues with resistant bacteria, like MRSA. Doctors thought it was "easier" to give antibiotics to anyone and everyone. Bacteria developed resistance over time, and now, we have few antibiotics that effectively treat these bugs, and the ones that do can be very toxic. The parasites are doing the same thing. It has proven for me to cost the same to do fecals, and I know that my horses are not going to have an issue. If they are positive, the vet tells me exactly what to give.
I have 2 horses, at 2 barns, one being a training barn, and neither is a "closed herd" at all. New horses get tested and treated, then added to the herd. All horses at each barn are on that barns schedule. I have yet to ever board anywhere that did not put right in their contract that the horses wil do the rotation O(previously) or the fecals(now). I am moving one of mine back to Va with me, and that barn does rotation. Prior to signing the boarding agreement I explained to them that I do fecals, and they are fine with that, and they get a copy. From what you are saying it would sound like you never do fecals, which means that your horse may never be free from parasite load, no matter what you give. That would make me a bit uncomfortable, but that is just me.
Oh, BTW, neither of mine has needed a thing since april. So we have gone the major season for parasites without any. I love that!