This is a very subtle tweak, but I beleive they offer an improvement. After covering all the unused jacks in my system with Cardas caps, I listened for several weeks. I then removed them all and noticed that the HF had a bit of an edge and the mids were a bit unfocused. I would recommend covering all the unused jacks, as this is a relatively inexpensive tweak.
As for unused XLRs, the same test yielded similar results, but the cost for Cardas XLR caps is several times the cost of RCA jacks.
Good luck!