It's generally a good rule, though sometimes the people who take the risk and go YOLO are the ones who win big, but of course the opposite is also true. I'm a big believer in bitcoin, but I'm not a big believer in people and those are what we need for bitcoin to push through and become very succesful, so I only invest a little of my own money and try earn bitcoins in other ways as I know it's not a foolproof investment. Saying that though, you could invest all your money in bitcoins now and sell as soon as they double or something, which I think is likely to happen, but it's not guaranteed and of course is a risk. Whether you take that risk is down to you to make.