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 :shifty::shrug:.
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.