I tried to use different surfaces to make tiles flat. I tried wood, plywood and drywall covered with canvas. In the past when I was working with very thin wood surfaces I had problems with warping corners or deformed pieces. The wood surface absorbs humidity from the clay and starts warping, therefore the piece warps too. A rigid surface is the best choice. I had better results with 1″ Plywood or drywall.
I think everything depends on the scale of the piece that you are doing. Small pieces can be very flat and dry evenly when you place the piece on rigid metal store rack surface. Laura Reuter wrote a great article about it.
The pieces below were made using four different clay bodies including porcelain. I used a drywall surface but I set an other drywall board on top to apply weight.
1. Rigid surface. Plywood 1″ thick or drywall.
2. Score in the back.
3. Always put plastic on top and bottom.
4. Large scale pieces are better to make on a big table, and if possible only move it to put in the kiln.
5. For making a relieve piece, put little weights on the corners to prevent warping.