Matt has it right, except that I disagree with this.
ORIGINAL: MTK
Don't fret the foam cut so much. A simple way to do it is to mark a center line around the foam block and place the templates on the line, root and tip. Make the cut as needed.
Good luck
The first foam wing I did was with a core that was cut by a guy using this method. He used pins and double sided tape to attach the templates to the center line of the foam block. The weight of the hot wire pushed down slightly on the template as he cut them, and the result was that the thickness of the two wings varied by over 1/32". The next set I cut myself using the method shown in the picture below. The trick is to insure that the top and bottom of the foam blank are perfectly square to each other. I use a couple of 2"x2" aluminum angles to skim cut the blocks if they are not.
This method works well for symmetrical wings. Since the block and the template are resting on the work surface, there is no distortion. Once one side is cut, turn the block over and cut from the other side.