The simple rule is "buy at the bottom and sell at the top". But i don't think this applies to all coins, it only applies to solid coins like bitcoin, ethereum and the likes, because they have stood the test of time and would surely find their way back up. But most of these new coins, if you buy at the bottom, the price might still go lower and keep you trapped there. so it is better to wait and see a clear move up before you get on the train to avoid fake outs.
Who says it's bad? The problem though for those who FOMO is that they don't know when to exit and take the profit because FOMO won't last and the price won't go on parabolic rise. That's why many noobs got burn during the last 2017 to early 2018 because they think that the market will bounce when all signs are saying that we are going on a downhill. FOMO is good, but you have to have some semblance of control and always keep your emotions check.