And keep in mind that taking 20 takes 20 times as long as just trying once. So if you REALLY need to succeed right away, then taking 20 isn't going to work.
But anywhere where they player could try, fail, and just try again ad infinitum, they can take 20 to save some dice rolling.
If time is important, or if something bad could happen if they roll poorly, then you need to actually toss the dice.