Can statements in the Book of Proverbs be taken as: God's promises, sayings that are usually true, sayings that are sometimes true, or sayings that have some truth in them?
Proverbs is wisdom literature: it comprises a number of wise utterances that give a general illustration of how the world works.
Generally speaking, there are two hermeneutical errors committed when reading Proverbs.
The first is to treat it as a collection of promises, e.g. "Train up a child the way he will go, and when he is old he will not depart from it." If this is a hard and firm promise, what do you say to good parents whose adult children have nonetheless come off the rails? On the whole, this verse is generally true: children learn wisdom from their parents and apply it when they get older.
The second is to treat Proverbs like case law, but if you do this, then you will end up with instances where two proverbs are formally contradictory. Drfuss already pointed out one: Prov. 26:4-5. Do you answer a fool, or don't you? Well, obviously, it depends on the situation. Sometimes the situation requires that you answer and refute the fool's foolishness; sometimes you just need to smile, say "Thank you for sharing, brother," and move on to the next person. The verses are wisdom, not law; indeed, it takes a certain measure of wisdom to decide which principle to apply in a given situation!