|Many pattern pieces are placed on a fold for cutting. I've found that no matter how carefully i try to smooth the fabric layers, the lower level is never completely ripple free. To make sure both halves are symmetric, I do the following:
1) Establish the fold line and make it as smooth as possible (avoid ripples - ripples on the fold indicate that your fold is considerably off grain).
2) Lay the pattern piece on the fold and pin it to the fabric. Make sure to pin down the folded edge, not just the cutting line.
3) Using a rotary cutter, cut around the pattern.
4) Remove all pins around the cutting line but leave the center line pinned.
5) Very carefully flip the whole piece over (still folded, still pinned down the fold line). Smooth out the upper layer of fabric (formerly the bottom layer) by lightly moving your hands from the fold line to the cutting line - try not to stretch the fabric, just get rid of the wrinkles.
6) Very carefully flip the piece over again. You will see that the bottom half is now bigger than the upper half in some (or all) areas. Trim the excess away and voila! Symmetry is yours!
Note: I've only done this with knits that have some body to them (cotton and a rayon/poly blend). The fabric tends to cling lightly to itself, which allows the 2 halves to stay together during each flip. I'm not sure how well all this flipping would work with something very slippery like slinky. You might need to pin the layers together before flipping it the final time.