Sorry, I'm in the Eastern US, so things may be very different
How big is her grazing and how well is it maintained? Meaning is the pasture rotated or dragged on a regular basis?
If a horse is in a large area that isn't overgrazed, their parasite load may be small. If the grazing frequently has standing manure and the horse grazes over the top of manure, well, then, yes, you probably do have a parasite problem.
I am really, really particular about dragging and rotating my paddocks, currently I worm 4X a year, each time before moving the horses to a clean paddock, and my vet tells me *that* may be too often to deworm given the paddock maintenence.
If you're still worried about adverse reaction to the wormer, I would still do the fecal floatation. It only costs about $20 US, and you can drive the sample to the vet clinic or lab yourself.