I haven't used the Derwent WC pencils but I do know that anything involving water should be used with water colour paper
It stands up to pilling and warping and will allow you to play with your blending better. (You can buy tablets of it at a local craft/art store and just cut small pieces for your cards and this will save you a lot of frustration than can happen later on when working with water and whisper white).
I used to use WC pencils but know mostly work with Tombows and re-inkers but I do still pull them out occasionally