Tension in the fabric is as important as tension of the thread.
Taut but not too tight is about right. When on the rollers with the rollers locked and ready to quilt, the fabric should deflect with the weight of your hand resting on it, but not much. You don’t want to bounce a quarter like on a soldiers bed, but you do want it firm, it should look smooth.
Sewing techniques can lead to tension issues.
If the fabric is pulled through a machine, or the machine presser foot tension is set differently between seams. The seams created can be “tight” or “loose”. On a quilt back pieced from long lengths to create a stripped effect, sewing improperly can lead to huge problems when mounting on a long arm. I also think sewing on a dry warm day versus a wet humid day can cause some of this. But so can sewing when you are upset with your husband and are working out your tension by escaping to the sewing room. Perhaps even moving from a kitchen table at the beginning of a project to the teensy side table next to the couch can matter (especially when sewing long straight strips.
It matters how you put a bottom on the roller bar
Lots of the issues mentioned above can be overcome by an experienced long-armer. Of course, I didn’t know this being so new to the game. I had to call in my guru, Laurie, who helped me laugh off everything and gave me tips to sort it out. Basically to overcome the tension in the seams issue, it helps to mount a pieced quilt bottom so the longer pieced seams run parallel to the roller bar. If you don’t and the seams roll around the bar, then the fabric essentially double layers wherever the seams overlay each other as they wrap around the bar. This means that the flat areas of fabric can’t lay flat. You’ll end up with tight seams and loose areas between the seams. The loose areas will wrinkle, pucker, and make your life miserable since you will not be able to get even tension on the quilt back.
Simple rule, on pieced backs, mount the back so the long pieced seams are parallel with the roller bar.