I've seen it before. You can cover them if you want. It won't make a difference. The roots above the medium will develop like a stem-like skin, in the same way that tree roots do when they protrude above the soil.
Perp Reviewed by Perp on . clones roots appear to be growing above the rockwool cube? I'll get some pics later. I have two clones i took when i topped my WW. They are about 8 days old and in rockwool cubes. They look great, but on one of them appears to have roots growing slighty above the rockwool cube, they go down into the cube. The other does not. Neither has roots showing through the bottom of the cube. Rating: 5