Stockinette stitch curls. It might block out if you made it of wool rather than acrylic, but curling is in it's nature.
If I make something from an acrylic blend that won't hold blocking, I don't bother. Seaming takes care of the curling and acrylic doesn't really block.
I do block Fair Isle sweaters, lacy things and wool things if I think they need it. If anything else looks ok the way it is, I don't bother.