Ok. Sheet as backing. Quilt Police would
. Fortunately they've been bound and gagged.
Rule of thumb is best outcome if front and backing are same or similar fabric, same fiber content and both either prewashed or unwashed when assembled. Logic says that makes for a similar shrinkage rate when the finished quilt is washed.
So,
ideally, you prewashed all square fabric beforehand. The sheet is 100% cotton. Then you follow the suggested quilting spacing for the batting. But, things probably aren't ideal. Sheet is likely the standard poly cotton blend, has been washed as slightly used. Squares for the blocks, maybe some prewashed, the rest ready to shrink to their hearts desire the moment they hit water. You are likely to possibly get random shrinkage on the front as you've used lots of different fabrics, different makers, different greige goods.
So, my remedy would be to use more quilting then necessary. This will help control any uneven shrinkage, making it more evenly distributed. Probably your best bet for the happiest outcome. It's also why crazy quilts and hodgepodge quilts have denser quilting. Everything stays in it's place.
Warm and Natural is probably the most popular quilt batting in the US. Consistently good results, affordable (sort of), nice generalized weight. It's extremely popular where I am.