I was told in my copic class - and it was taught by a certified copic teacher who was trained by Marianne Walker that copics are supposed to bleed through the paper - otherwise you aren't colouring correctly such that you can blend. I was told to use wax paper underneath my cardstock so that the copics didn't stain my table due to the the bleedthrough, I use Neenah solar crest white, and I like it quite well. The LSS that I took my copic class at has some sort of copic paper that is even better - more bleed through but easer to blend. I don't know what kind of paper it is though.
SU Whisper white is no good with copics - to much seepage of the ink from one area to the other - and it has a coating on it that makes it great for gamsol magic - but not for copics. HTP.