Generally, for quick and dirty, it probably doesn't matter provided the RC formula with the best MSE is used and the player is closer to the league mean than the extreme. I often speak in terms of RC for the sake of discussion. Why is this distinction important then? Well generally speaking because it's tough to justify continuing to use a less accurate method when better ones exist (and as sabermetricians are gravitating toward the better estimators our pet websites, though lagging,will have to follow in order to remain relevant). Also-and probably most important for this discussion- quite often advanced defensive metrics like UZR are criticized for their LWTS component. It's simply not a valid criticism from a run estimation standpoint.