where you place rootless plants strongly depends on the humidty in your normal setup. What works well for Josh doesn't work for me. The humidity in my terrarium is too low for rootless seedlings. I place them in a closed plastic bag untile they have rooted.
@Rob: How high are the light levels in your N. argentii growing area? My plant is grown in a very bright spot and seems to dislike it not getting bigger in size (which of course could also be caused by other reasons).