I mean no disrespect to anyone but I am hoping to clarify a couple of things.
The confusion may seem to be the use of ounces for weight and the use of ounces for volume. For volume measurement, it is usually referred to as fluid ounces. One fluid ounce of something does not necessarily mean one ounce (by weight) of that item. According to Wayne Glissen in the Professional Baking book that FK is referring to, the only exception is water, eggs, and milk.
Ounce as a volume measurement is typically just used for liquid.
I suspect that what FK is referring to is that if you sift in a measuring cup with fluid ounce measurement, once you reach 10.5 oz (by volume which is fluid ounces), it may not necessarily be 10.5 ounces by weight.
However, in Rose’s books, anytime it is referred to as ounces, she means weight. So, like Charles was saying, whether you sift it before or after, it does not matter because the weight remains constant. So if you are weighing before you sift, you weigh 10.5 oz of flour, then you sift it. After you sift it (all of it, everything that has been scaled), the weight of that flour is still 10.5 ounces.
If you decide to sift it beforehand, sift your flour, put a bowl on a scale, set to zero (tare), start adding sifted flour to the scale until you get to 10.5 ounces. In both methods, the weight of your flour will be 10.5 ounces.
Where sifting before or after matters is if you are using volume measurements, in which case, Rose’s recipes typically measures flour by cups. If the recipe says sifted flour, then you sift over a cup and then carefully level the cup with a knife. If the recipe says unsifted flour, then you put flour on the cup, level it, then sift it. the weight of these two cups of flour will be different because the first one is aerated and therefore less dense than the second cup which will be heavier.
Rose has a video about weighing flours that could be helpful: http://www.realbakingwithrose.com/2010/02/baking_magic_tips_weighing_flo.html
Hope I didn’t cause more confusion.