This is certainly the conventional wisdom, but I'm not entirely convinced.
From 18 Monk to 20 Monk, you gain +1 AC, +1 hit, +2 damage. You also gain 10/epic DR, the capstone boost to Ki, and Shadow Walk.
It seems like losing all those benefits and gaining access to scrolls of Stoneskin, Shield, Divine Power and Divine Favor would be well worth the tradeoff.