Yes, they are also called terminators, or termination plugs. I use one on the SPDIF out of my CD transport and the BNC input of DAC. They made a difference (at least in my system). The nice thing is they are fairly cheap.
As far as the RCAs, make sure they are actual shorting plugs, with a resistor, not just caps, like these:
https://www.hificollective.co.uk/accessories/rca-shorting-plugs.html
As far as the RCAs, make sure they are actual shorting plugs, with a resistor, not just caps, like these:
https://www.hificollective.co.uk/accessories/rca-shorting-plugs.html