I am of the opinion that if you are using your FL sizer to partial size, the shoulder moves forward more from that, than from firing. I, when doing this, always maintain the shoulder datum. That is to say that the sizer die "bumps" the shoulder back to its position after the first firing. What I am trying to say is that the sizer die pushing the sides of the case in causes the brass to flow forward, ever so slightly. Imagine pushing in on the sides of a balloon. I use either Stoney Point or RCBS equipment to measure the shoulder datum. To the original question, I do use a universal decapping die as I normally have the expander ball as high in the sizing die as is possible. I believe it allows for less run out.