Silk or cotton pillowcase?
updated by @mama-jme: 01/13/15 09:40:17PM
Loose hair is what you want. It is from these that dreads are eventually formed. Maybe a cotton pillowcase is better because it seems, to me at least, that it would cause more hair to become loose and then tangled back up into the dreads. I wouldn't worry if your using a silk pillowcase though. I'd say its more the act of moving your head during the night that helps dreading, rather then the pillow texture.