From the mouth of Nison -- paraphrased: usually it is a good idea to wait until the signal bar closes. However, let's say you are looking to get into a long trade on a bullish engulfing pattern, and there happens to be another candle some bars back, say a rising window or a southern doji to indicate a support level close to the point at which you want to get into the trade, it may not be a bad idea to get in. As with any trade, the candle indicators are not an absolute -- rather, one must consider the the immediate candle in the context of other candle indicators (preceding candles, the same chart on different time frames, etc), and even other "western" technicals to analyze the risk-reward aspect of the trade.
As with any technical indicator, relying on them mechanically, without considering the context, one's results will be haphazard, at best.