I have been using one filter paper disc at the bottom and one at the top and then a stainless steel mesh disc on top of the filter at the top. I have not been finding any negative tangible difference in taste. On the other hand the shot usually runs faster than if I don’t use filter paper so I assume that it’s helping the flow through the puck.
My main reason for doing it is because some studies apparently showed that it cuts down significantly on the fats that are transferred into the espresso. I have started using two filter paper discs at the bottom for this health benefit to enhance the filtering out of fats even more and I am not really finding any tangible diminution in the taste.