Generally water taken on the hill in wild country, with no sheep or cattle grazings has seen me Ok for many crossings.In lowland areas and nearer the East coast, then water sources are not reliable unless taken from a domestic or public source, or filtered.

You will find you will get a variety of responses to this one!
A lot of Challengers will happily take water from any good source and drink it fresh. If unsure, a good boil for several minutes should do it. Obviously if you're in the midst of farm land, particularly as you get closer to the east coast, good water sources will be more difficult and perhaps getting your bottle filled at a pub or cafe or buying bottled is safer. Water in commercial forestry may also have problems with pollution.
Personally? I use a filter bottle, Water-to-Go, for my fresh drinking water and I use Aquamira treatment drops in the water for my meals, because the cooking system I use is not designed to boil for long periods (Jetboil Flash). Why? Because knowing my luck, I would be the unlucky one caught out, for a few grams why take the risk?
Others, as I say, don't bother.
Personal choice.

I prefer to (Sawyer) filter just to be safe.

If the water looks iffy I'll also boil - and even add a steri tablet if it looks REALLY suspect.

Each to their own, many are happy to use untreated water, I prefer to err on the side of safety.

See you in May!

For years I never filtered or added tablets. However in April last year doing the SNT I got struck by a horrendous stomach bug. Since then for me if its not tap or bottle I add tablets

Sorry to hear this happened to you as well, Steve. The dry winter is to blame, is what I heard. I've never filtered water here either (northern Sweden), but I've heard of bacteria being found here as well.

I had planned to buy a Sawyer filter before a trip to California in September (fingers crossed!), I may as well get it now.

Personally I carry a big old filter. But there are lots of very small ones out there now. Carrying one doesn't mean you have to use if the purist in you will be upset. But sometimes - especially if it is a dry year - you can get water from dodgy places you wouldn't normally use.

It's down to where you are.
In the West and centre, away from industrial areas, forestry and farming, as long as it's moving, I've never bothered with a filter.
But nearer the East, it can be harder, especially if hot.
I always carry some purification tablets for emergency, and either a Sawyer Mini, or last year a Katadyne filter.

I use aquamira (chlorine dioxide) drops most of the time here in the US. Occasionally need a filter for scummy/muddy water in Grand Canyon, Utah, Texas. I plan to use aquamira in Scotland. Drinking water only. I boil water for coffee/tea and rehydrating dinners.