In my experience, the post sizing ring does deform or otherwise destroy my cast boolit (i.e. size) and I would not personally recommend them to anyone. I suppose it actually would depend on what size your cast boolits are, and what size the Lee FCD post sizing ring is, in the inside diameter.
From what I hear, Lee is not consistent in the sizing ring inner diameter, and this could explain why some say that the sizing ring does not size their boolits. I have only tried the one FCD in 44 cal., so that is hardly a representative amount of use to draw a positive conclusion from. I do like the crimp it applies so I simply popped out the sizing ring with a punch. That is enough for me to want to stay away from them, but that is a personal decision for me alone.