I weigh my pack by lifting it with my hands and strapping it onto my shoulders and hips. My body then tells me, roughly, if it's heavier or lighter than last time I put it on.
If I can find a personally-acceptable way to cut down on weight, then I do so, but I don't feel the need to know what the results are in numbers - only in strain on my body.
PS. I think I've weighed my pack on scales before a walk about 3 times, but I can't remember how much it was on any of those occasions.