There will be about 50% of the quilters who disagree with this advise, but after spending money and time on fabric, launder it first. I don't use detergent, but do use color catchers. I put the machine on "hand wash." The reason for all of this extra work is to prevent shrinkage after the quilt is finished. Cotton will shrink! Then it pulls against the thread and weakens the fibers around the threads. Colors will bleed onto each other too. This helps prevent that from happening the first time the quilt is washed.
Shrinkage happens with the hot pounding dryer. If you plan to line dry your quilt forever, you probably could skip this important step, but after all the work, money and time involved, it is better to be safe.
Your steam iron will not shrink the fabric. It is the pounding that shrinks it. Steam irons stretch the fabric.
I have been fortunate to find what I need with discounted prices. It is still an expensive quest, but so rewarding, making quilts. Blessing to you, and jump right in.