This saying is only said by bagholders who are trying to justify their bad investments. If a coin is only going down, then selling it as soon as possible is the correct action to minimize any future losses. Taking a loss of 10% is a far better option than holding a useless coin in to oblivion.

When BCH was first forked, it was trading at 0.2 BTC. I, and most other smart people, dumped it immediately. If I had held on to it and were planning on selling now, I would have lost 75% of my profits. If I continue to hold for another year, I have no doubt I would lose 95% of my profits.