If you have a hat that's a hair too small, or a sweater a few rows too short, you may want to try a more aggressive blocking technique:
Rather than pinning and spraying the piece, submerge it in a bath of room temperature water.
Remove to a clean towel and roll it to get out excess water. Don't wring or twist the knitting while it is weak and wet! Be gentle!
Hand block the damp knitting to the desired size and pin it into place. Let it dry thoroughly.
Good questions! Your sweater will continue to grow and change a little bit, stretching and loosening up as you wear it. It should not shrink unless you wash it in warm water or machine dry it.
As for the scarf caught in a rainstorm: after you've dried off, towel dry your scarf and lay it out flat on your bedspread to dry fully. Because the fibers are weaker and stretchable when wet, don't hang it over a chair or from a clothesline.