Actually, for this type of application where you're cutting something out in 3D that's symmetrical along the long axis, it's much easier to just get into the habit of mirroring everything you cut out. The worst mistakes I make is when I get complacent and/or try to outsmart myself. If every part is mirrored, even if it doesn't technically need to be, then you're safe.
With the cost of materials and cutting time, imagine you use your last piece of MDF and wait an hour only to find out that you biffed it and Home Depot is now closed. You were planning on doing the glue-up tonight so it would be ready tomorrow, but now you can't and it won't. Plus, tomorrow night you were planning on using penetrating epoxy so it would be ready to paint it Friday night so you could take it to a costume contest Saturday night. Now the whole thing is shot and you missed out on the Grand Prize, a charter in the British Virgin Islands. Spiral = out of control, all for want of an extra square foot of MDF.