I've found that once I block a sweater, it stays that way. After that I wash it and lay it flat to dry, maybe stretching and shaping a bit. I don't block pieces, generally. Not because I know best, but rather that I'm lazy
Blocking tends to relax the stitches and even them out. They don't go back to "newly knit" after it's washed.
Some things, like lace, may need more care and attention.