I just see the 20 buyins as a goal to move up. Ive just moved to 25NL from 10 NL. I have 19 buyins. Moved up at 20 and ive taken some horrific beats since but I feel im playing well (and profitably) so im sticking at it. If I fall below $350 or so Ill drop back down to 10NL, and then I wont move back up until $500 again.
Those 20 buyins allow you to lose a few before you move down, and then still have a good sized bankroll for the limit you have to drop down to if the worst happens.