I have had no problem with the temp. spray adhesive, although I would caution against using with a longarm. Are you saying that you still get pleats sewn into your backs on your longarm system? Is it a three rail system? Maybe you are not snugging your back enough. You must make sure that each rail is slightly taught. If you are getting pleats towards the side of your quiltback, maybe you need to get something to put tension on the sides. We have spring clamps connected to a velcro strip at the shop. These are clamped on the sides, after the rollers are tightened.
If you are talking about a wrinkled back prior to quilting, and you don't want to take the time to iron, we put a few drops of fabric softener in a spritz bottle along with mostly water. After we pin the back to the rollers, we give it a spray and smooth it out. It is cheaper than the store bought wrinkle releaser on the market.