Natural fibers should typically be blocked, particularly when knitting garments. It ensures that all of the pieces are the proper dimensions which makes sewing, for instance, sleeves onto a sweater easier. Synthetic fibers usually cannot be blocked, so if you are knitting a form-fitting garment out of an acrylic yarn make sure you knit it precisely to the measurements that will fit your body (or the intended recipient's) so that it will fit well without being blocked to size.
Scarves, hats and socks usually don't have to be blocked, but you can make that decision when you see the finished piece. If you're pleased with it then don't sweat it. If it looks a bit curled around the edges or a bit puckered along one side, by all means block it. For example, many people prefer blocking an Entrelac scarf because it really helps straighten up where you have worked the short rows. It really makes the woven effect "pop".
Some people say you should never block ribbing, but that is a myth. Granted, there are ways to properly block a ribbed piece so as not to effect the integrity of the rib pattern, but you can block ribbing none the less. It's a matter of preference.