There's no harm in weighing the amount of oil you put in your diff to try and get it the same each time. XRay provides the weights you should use in the manual somewhere.
Most of us build our diffs so infrequently (or pick up a second one filled with slightly harder oil than what we run normally) that we don't remember the weight or have the manual around to look it up, or don't have a scale, so we just eyeball it.
I would say, weigh it the first time around if you've got a scale handy so you have an idea of what what the volume should look like, and then eyeball it going forward. It's less oil than you might think.