I think bearding depends so much on several factors--the tightness of the fabric weave, the density of the quilting (lots of overlapping) the weight of the thread (lower # =bigger needle). I will usually go ahead and use 2 black bats if I have black background on top and bottom--I figure bat isn't perishable and I can save the neutral or white for something else and just not worry about it that way.
P.S. if you are using 2 bats to get lots of quilting definition, I found great success with one bat being Hobbs 80/20 black bat and one on top of it being the Quilter's Dream black poly from Connecting Threads--not always in their mailed catalog so check their website or give them a call.