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.
I don't see this post have anything to do with FOMO
FOMO and the golden rule of trading are not the same, looks like OP has missed the point here. Anyway, the lower the trade has bought the altcoin, the more profit will flow the balance of trader. Money management directly depends on the entry-exit points, so buying low turns to be the advantage of trader.