Not Loosing Money is more Important than Making Profits.
This is by far the important rule, people when they think of trading think of all the money they are going to make not understanding that if they actually try to make as much money with their small capital they will soon lose it all, the most important thing to learn is to trade defensively and not lose money, it is obvious this way of trading will not produce the huge profits everyone wants but over the years and the decades it is the only way of trading that will make you wealthy.
The main target of trading is to make profits, not the other way around, so stop loss is a common feature for traders but not for long-term holders, they prefer to buy the dip rather than sell at a loss because we know bitcoin value will increase eventually. The idea of trading is like gambling where people who don't know to predict the market will lose but the people who lose money are those who cannot be patient waiting for the price of bitcoin to recover, yes it may take weeks, months, or years but it will someday.
A different situation for trading altcoins, better sell it before its value diminishes for good, hence do not trade shitcoins, stick to some major coins in the market such as eth and ltc.