I'll second that!!
In addition to above, when I first picked up my old mare they told me that she'd probably need to be wormed (she was a bit skinny) but I took a sample to the vet before I wormed her to find that she had no worm burden what so ever.
I lot of people worm their horses regularly, which in theory is a good practice, but take into account the chemicals that are being pumped into your horses body when you worm them, if they didn't need it, would you still do it??
I will worm my girls religiously just after bot season as I know that they ingest some of the little blighters, but other than that I will send a sample off during spring to get tested. Haven't had a problem yet