Over at Crafting a Green World Wenona Napolitano wrote a great how-to on how to craft your own handmade paper! I used this to make some plantable paper for my soap tags (it's like you will be getting a free plant with your soap purchase!) She suggests to wait until your paper is in the molds, but I just threw them in the slurry pulp mixture AFTER it was blended. You can do what ever you think is best.
So check out her how-to here How-to: Handmade Paper