Most longarmers ask that the top be pressed, threads be trimmed, etc. before they get it for quilting. There is nothing magical about the longarm that makes the seams lie flat. However, if the back is wrinkled, I have found that I don't have to press it (except at the seams) because I can easily get the wrinkles out by spraying very lightly with water when it is stretched on the frame.
I press at every step during piecing, again when the blocks are put together, and a final press if necessary before quilting.