I wouldn't steam poly bat or do any tucks--think that would be obvious with a lone star. I'd go for misting the entire quilt (be careful if you think there will be bleeding of dark into light) and then do as Feline says--block it. I'd probably set something heavy on the middle until it dries.
When I get a quilt with lots of bias cut edges (like a lone star) I will go through first and baste the entire quilt before I do any other quilting on it. Have found it helps to keep things stable. I like Jamie Wallen's basting method of doing a long stitch in a big stipple (I use a noxious neon pink thread so easy to see to pick out!).