That's what I do, on a lot of the foam unfolds they are only going to give you one half of the symmetrical shape. I lay the piece down,trace it, then flip it over and trace the other half to create one large outline. Some shin (and some bicep) unfolds are especially notorious for having only one side of even center pieces, so if you cut each side separate you'll end up with a seam running down the dead center of your entire part. If you look at the bicep in the right of the pic below (ignore the red circle), you can see that there's no seam down the center because all of those templates were flipped like you did for the faceplate. You can even see the pen mark in the foam towards the top to help me align the flipped template.
http://www.mediafire.com/convkey/26a3/9xpdzo541kh9erx4g.jpg
Less seams is better, but just make sure that the piece your flipping is truly a straight edge; even lightly curved edges usually come out better if you cut the sides separate and then glue, because the joint is supposed to have a bit of curve to it.
awesome so im on the right track now here is another question do you shape and glue pieces as you are cutting or do you cut everything shape than glue piece by piece?