The presence of a seam can be a concern if the fabric used is translucent enough that the light passing through the main fabric bounces back off the fabric of the seam allowance, and becomes visible from the front in the form of a
line of brightness in the projected
image. Light behind the
cyc can
reveal seams (exit lights, hallways,
etc.), and stitch lines can sometimes pucker slightly causing wrinkles that are nearly impossible to fully hide. If your gig is one that really wants 'perfect' (like a corporate event), then pony up for the seamless
cyc (and pass the cost on to the client). If some imperfection will be forgiven (like a student project), then seamed will likely be cheaper.
20' high seamless is easy, though expensive, looms exist that are wider than that, one merely hems up a 50 yard piece from such a
loom to get the 20'x145'
soft goods you mentioned. But, with so many more threads to manage and possibly break, getting flaw-free fabric is more of a challenge, and the mills charge accordingly.
