The older your house, the greater the chance there's an area that needs scraping. A previous paint job may have begun to peel or crack in some places. Windowsills and sash frames may have chipped, or the old paint may have "alligatored" into a maze of cracks.
If you find these conditions, scrape them gently to remove the loose particles, then sand them smooth to blend with the area around them. If you get down to bare wood on woodwork, prime the spots before you apply the final coat of paint. If it's impossible to blend the scraped areas with the nonscraped areas on walls, go over them with a light coat of drywall joint compound. When walls are dry, sand them smooth, prime, and paint.