I have issues with symmetry, so I always pattern half of whatever Im doing an flip it. It saves materials, time, and contributes to better symmetry when done right. Here is a little trick I do when I mirror a piece.
First I tape the piece where its going to be mirrored onto a piece of paper and then flip it and trace. Then cut out and you have a complete piece. I only do this on places that can be combined.
Good places to use this in order to avoid a seam where it doesnt need to be include....
Front Shin
Back of Calf
The arm (this can be a whole tube in some cases! Only on seam needed!
Top of the muzzle
Between the eyes
Top of the head
Back/ Front of the Body (although these are big and its usually easier to flip them!