Search PubMed for Robert Derlet, MD; he's a UC Davis professor who has spent decades researching water quality in alpine areas with a specific interest in whether or not it is necessary for backcountry travelers to treat their water. Long story short, his conclusion is that in alpine areas, experienced backpackers probably do not need to treat. As many doctors will tell you, personal sanitation is WAY more important. Here's a good interview, from REI no less:
FWIW, I'm on the east coast, and to me there is a big difference between a remote spring and a stream in a developed/high-traffic area. The first I would drink without filtering, the second I would probably filter. On shorter trips when I know the area and have a good idea of water sources I'll be using, I feel comfortable leaving my filter at home. On a thru hike, or any trip in an unfamiliar area, I bring a filter.
Also, re: iodine, the human body only needs a tiny dose of it, and most Americans get plenty in table salt. Too much iodine can cause thyroid dysfunction, goiters, etc. Though I can't find much agreement on how much "too much" is. Also, iodine is not recommended for people allergic to shellfish, pregnant/nursing women, or those with thyroid conditions.
Like Rusty, I was raised on well water, not municipal water, so the idea of using chlorine in mountain streams seems pretty backward to me. Sawyer Squeeze FTW.