But, as I said, that's the scientific response. Two practical considerations: One, how are you going to keep the dog from drinking from a lake or stream? Maybe you keep him leashed all the time (and, if so, good for you!), but even then you'll have to pay very close attention indeed to ensure he doesn't dip his snout into a creek for a quick, cool gulp. Moreover, my research indicates that most dogs can fight off a Giardia infection without too many extreme effects. Symptoms are moderately obvious (ranging from light-colored, soft feces to explosive diarrhea) and treatable with several antibiotics. So Giardia is not fatal for a dog, and not even particularly harmful. Actually, I've had Giardia myself, twice, and while it's not something I'd pay to have again, it's not the worst thing in the world.
Secondly, there's an assumption that Giardia (or other nasties such as Cryptosporidia, another water-borne parasite) is everywhere. The conventional wisdom, at least in my part of the world, is that Giardia is more widespread than it was 30 years ago. But it's impossible to prove both assertions. Some studies suggest that Giardia is more common in the average municipal water system, even after treatment, than in your average backwoods stream.
Generally speaking, then, I think ticks and other insects pose a greater risk to dogs, and even humans, than does Giardia or other water-borne parasites. Take precautions to protect your pooch with tick repellents such as Bio-Spot, and don't agonize too much when you catch him stopping for a backcountry slurp.