I would definitely include the boning. It is not difficult (use the seam allowance of the lining layer of the corset/cincher to create a couple of channels beside each seam), and it will give you a much cleaner appearance, provide some support to the garment, and, importantly, provide some support in the garment where the tail attaches. Without boning, the weight of that tail hanging off the back will pull the cincher down, making the back of it saggy, baggy, and uncomfortable.
Pictures and tutorials from my costuming portfolio at Syagria's Cosplay Site!
Star Trek Original Series Uniform!
Original Creation for AB13
Most Recent Con:
PAX East, Anime Boston 2013