Always. When backpacking, I have used filters or iodine or boiling. In my BOB, I have means to do any or all as needed.

That said, if I have none of these available and I am lost in the wilderness I'm drinking the water I can find unless I know it to be bad, especially if I expect rescue within 3-4 days. It takes days for most water-borne diseases to show up. I could be dead of dehydration before that if I drink nothing at all...