I don't press the back anymore, unless it's pieced (and then I just press the seams as I am going). After I load the back on the longarm, I roll it so that only the bottom part of the quilt is visible. I mist it lightly with water while the quilt is stretched and the wrinkles just vanish. I let it dry and advance to the next portion of the quilt, repeat until the top part has been misted and dried. Actually I'm not sure that it's important to thoroughly dry the quilt at each advance, but I usually do. Keep in mind I'm not a professional longarmer, so these are my own quilts, charity quilts, or friends' quilts. A professional might be reluctant to put water on a customer's quilt, and might not want to take the time to let the quilt back dry with each advance. However I just did this with a wide back that I had washed, then dried over the banister, so it was full of wrinkles, and the longarm mist/stretch took them all out, as if by magic.