I live in Wisconsin, so I can burn wood... So, whenever I can make a campfire and boil water - I'll go for it, and then it is free. Or I'll cook with untreated water in the evening - same thing, it's free. Or I will boil water for coffee - it takes same amount of fuel to boil it, whether it's treated previously or not (and coffee is my lunch drink whenever I can have lunch).

So, essentially boiling is my preferred method of cleaning water whenever I have time to do it. And if it would "cost" me a bit of extra fuel, that's OK with me too.

But I do bring variety of backups for when I'm short of time, on the move and out of water now.