The way I've always understood that to mean was not about playing the open, it was that the open until conciliation reflects the behavior and opinion largely of the "crowd": retail investors who placed a trade before going to work, or last night, and the majority of these types of traders around the world while the market was closed, whereas the time period before closing reflects moreso the behavior and opinions of the pro traders and institutions, either going flat on the day or making a decision one way or the other to hold or not.
Well to answer your question, Cheese, I trade both. Triggers occur as swings through points of balance or from initially unbalanced conditions which typically occur at the beginning of the day.
It's generally regarded that opening provides more volatility trades while closing provides more volume trades.