I will always block items where size counts, ie sweaters. I'll also block lace items because blocking lace brings out the pattern. I don't block items like blankets or scarves.
Blocking is kinda like heat setting your hair (like with a curling iron). When you get the yarn wet, and manipulate it into the shape or size you want, then let it dry that way, it forces the yarn to retain that shape... like using a curling iron. But like hair, next time you wet the item (wash it) you will need to re-block it.
Blocking works best with 100% natural fibers or blends that contain some natural fibers. You can not block 100% acrylic, but it usually doesn't need blocking anyway.