For the first three washings of new towels, do not use ANY liquid, powder or sheet form of softener. If you do, the towels will become less soft and less absorbent.
When you first buy towels from a store they are filled with "sizing". This product, which is similar to wax, is used by manufacturers to keep cotton looking crisp for display. Fabric softening products can trap the sizing in the material which makes the towels less soft and less absorbent.