Pinning your fabric the right way is easy to learn. Done correctly, your seams will match up perfectly. I don’t usually pin fabric unless I’m matching seams. But I encourage my beginning students to do it whenever possible. Here's a few tricks to help you match seams.
Press seams in opposite directions.
With right sides together, nestle the two seam folds together.
Place a pin on both sides of the seam line (about 1/8" away from the seam).
Stitch, removing pins before the needle reaches them.
Press seam up.
Seams are perfectly aligned!
Pinning your fabric also makes sense when you're sewing long strips of fabric. This prevents your fabric from stretching out of shape as you stitch.