Julie,
Dryer sheets leave a residue on fabric that often stains or leaves the evidence of what looks like a stain. They aren't good to use on anything.
As for liquid fabric softener, that is up to you. If you are prewashing your fabric and you would like it to remain a little stiffer, the fabric softer will not help you out on that. It truly makes it softer. (they use it for bending wood, too!)
Liquid fabric softener will also help make you fabric a little more "water resistant." That's why it's not recommended for towels. On the other hand, I use it when washing wool garments because the water is more apt to "bead up" on it when in a light rain.
Hope that helps.
M