This is something I've personally been thinking about as well recently. I might sell part of my BTC at $50k. With part I mean something close to the money I invested in the first place, to make sure I never will get into negative territory again. In example: I invested just over €1k, which is now worth about €9k. If I sell €1k worth of BTC now, I still have the other €8k to grow further (although at a little less speed). If the price goes up, I send the €1k back to my savings account and am sure I won't lose money on BTC. If the price goes down, I can use my €1k to buy more BTC at a cheaper price.

However, I'm still not sure whether I should do it or not. It's also tempting to just hodl and see how high (or low) it goes.