The yarn will somewhat go back to its original shape when u wash it. But i do still recommend blocking whatever you create as it makes a huge difference. Usually after stitching the blocks together, the blocks will retain most of its position so you do not have to worry about it going crazy wazy on you. It might just shrink a little.
Whether the project gets back to its original shape also depends on the yarn type. Wool tends to have a memory to it so it will be easy for it to shrink back to its original shape. One method to help retain that shape is to dry your project flat instead of hanging (hanging makes your garment stretch). You might wish to steam/dry clean your garment instead of washing it to retain it's shape.