Greedy is one thing that we should avoid. If you think you are greedy, it is better to take your profit before you miss that chance, and you can't sell at the price you want or at the high price. But we can't deny that greedy will always come to us, especially if our target price is reached, but we don't place any order sell at that price.
If you are disciplined enough to know that greed will get you nowhere then you can be sure that you will be safe from possible let downs. One thing that I do believe a trader should possess is being content, content with what profits that he/she reaped and not minding the continuous price climb because he/she knows that the profit was enough. Market will not always be high, there will be lows and no one knows when is the low happening so you better get that profits before the low times come and you will waste another year waiting for another high.