+ 1 Jerry Adams.
Knowing your source and having a good idea what the likely contaminates (if any) will be, goes a long way toward assessment of actual risk, rather than worrying a stray drop will do you in. No doubt, there are some places where an abundance of caution is justified. There are also places where not treating at all is perfectly reasonable.
It's a little over a week since I got back from an 8 day trip on the Wonderland Trail. I drank some raw water every day, and treated almost nothing the last 2-3 days. So far, so good. (Incubation period of giardia is often cited as 7-10 days.)
Of course, most of this was water taken at higher altitude, often near a visible spring, in a National Park where stock and other domestic animals have been banned for decades. Rangers there still recommend you treat everything. I wonder how many of them do?
When I do treat water, I've never bothered to wipe 'dirty' water off threads or the outside of bottles. Never worried much about getting a bit of water in my mouth or eyes swimming in rivers and lakes. Again, so far so good.
Perhaps I've just had a good run of luck, playing the odds. If I got sick from something in the water, I'd prob. be a lot more careful.