I find the synopsis of Chapter 18 given by Sr. Joan Chittister in her commentary for today's passage immensely helpful. She writes,
Benedict implies very clearly in this chapter on the order of the psalms that a full prayer life must be based on a total immersion in all the life experiences to which the psalms are a response. . . . The Benedictine is not to pick and choose at random the psalms that will be said. The Benedictine is not to pick some psalms but not others. The Benedictine is to pray the entire psalter in an orderly way, regardless of mood, irrespective of impulses, despite personal preferences. Anything other than regular recitation and total immersion in the psalms is, to Benedict's way of thinking, spiritual sloth. Ours is to be a full spiritual palate. Readings may be shortened if situations warrant but the psalms never. We are to tap into every human situation that the psalms describe and learn to respond to them with an open soul, an unfettered heart and out of the mind of God.