That is exactly what gets most people recked while trading. You should take profit as they present themselves and not be too greedy.
Yes, we shouldn't be greedy in the market while we are making a profit because we should safeguard the profit aside, we don't know when the market will lose its value. A trader shouldn't be greedy for making a fast profit, this may lead to losing more value of your asset. So we should plan them wisely and we should react according to the market situation.
We cannot really avoid being greedy in this kind of market, it is obvious that if you are a trader that you are doing it because you want to obtain more money than what you have and that is being greedy, the problem is when your greed reaches levels that no matter what you do it is going to be impossible for you to fulfil your goals, you should always keep your greed in check otherwise you run the risk of greed taking control of your decisions and that always spells disaster for anyone that allows that to happen.