it's funny - i do just the opposite. When my backing is pulled tightly I spray it, and then i smooth the batting over it.
once the batting is smoothed, then i spray it and smooth the top over it. I do this with 505 on cotton fabric with quilter's dream cotton batting and it works every time. I would be afraid that by spraying the batting i would not be able to get my backing tight enough to avoid any lumps, bumps or wrinkles.
To each their own! :-)
lisa