You should always carry a little more thank you think you need (within reason) but for water, it's not a terrible idea to carry a water treatment method. I like Sawyer's ceramic filters unless I'm heading into the wilderness, then that becomes a backup filter for an actual purifier.
water treatment only works if water sources are on the route and naturally occurring water is more common than rest stops (with vending machines, fast food, bathrooms, water fountain).
Works fine on tap water too, but yeah, can't filter what isn't there.
If you're lucky though, you might encounter stations that sell fresh filtered water cheaply so you can refill your bottles without creating more waste, spending more money, getting exposed to more microplastics, etc.
The stations are pretty nice if you're traveling through areas that have them.