In the production of all my work a POL-C (circular polariser) is employed constantly on Velvia 50. POL-C is removed (use SKY1B) for early morning/evening imaging. It's an exceptionally useful filter for anybody to have (my sole POL-C is a B&W Käsemann 77ø). It's excellent at all but removing (that is to say I don't "flatten", or removeall spectrals from the scene) and adding the dramatic wow factor in Ilfochromes (but Velvia is still judiciously over-exposed a little for this).
An EV2.5 (1.3) factor is engaged when hand-held metering with a POL-C.