Is there a general rule to this for backpacking?
No, there isn't any general rule. It depends on a lot of facts, perceptions, and preferences.
I guess the water where you hike is unsafe to drink, and filtering is a reasonable way to make it safe. If you prefer to carry 5 liters of water in order to filter less often, that's what you prefer. If you prefer to carry zero liters of water and filter each time you drink, that's what you prefer.
For people who hike in the Sierra Nevada, backcountry water is safe to drink without treatment [Rockwell 2002], so the answer is different from the answer in your area where the water is unsafe. In the Sierra, where water is generally pretty plentiful, there is seldom any reason to carry any water on your back. Pack an empty Dasani bottle, and when you come to a creek, fill it with nice, safe water, drink it, and then empty the bottle back out before continuing on your hike.
Yet another possibility would be that someone hikes a lot in an area where there's a significant risk of getting sick from viruses in the water. In that situation, filtering won't help, because filtering doesn't eliminate viruses.
Rockwell 2002 - Robert L. Rockwell, Sierra Nature Notes, Volume 2, January 2002, http://web.archive.org/web/20051026030831/www.yosemite.org/naturenotes/Giardia.htm