i think it's true and know when to go out is very important and your gain or your loss depends on that.
That applies to longer term holding, and not so much to trading, which is more of a short term practice. In trading the entry points are as important as the exit points, because in a market as volatile as the one from Bitcoin, every few passing minutes may offer you an opportunity to quickly enter and exit with a profit. To add; in a rapidly changing market, you some times also have accept taking a small hit in order to adjust your positions to benefit from the changed market sentiment. If you keep sticking to your exit targets in case you don't take a hit, you'll be wasting precious time, and likely still be taking that hit later on.