Giardia can come from any warm blooded animal, including humans, and based on the amount of people hiking in the Whites, I always filter my water. There is no way to tell if there is Giardia in the water or not by visual inspection, and the disinfection chemicals are just too ineffective to work by themselves in cold water temperatures. Testing for Giardia is very expensive and takes some time, so it's not feasible to check all the water sources for it.
Sure, people can get lucky, the Giardia may not be present, you could be more immune to it than others, and there may not be enough to fully infect someone. However, I'd rather not take my chances, so I'll filter. In the winter I use a fiber filter, like the Katadyn Hiker or Guide (I need a new filter for my Guide though), and in the summer I use the WaterWorks EX (I don't want the filter element to crack in the winter), both with disinfection chemicals.