I scrape my mower decks every evening before putting the mowers away...well I at least check them...sometimes they don't need it. Sometimes they get done between mows or even during mows if the grass is tall and wet and it starts spitting clumps. This time of year its been a daily thing. I find that when I'm running the Ultravac on my Lazer it rarely needs scraping.
Some people spray the underside of their decks with products to reduce buildup. I never have.