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.
Bitcoin and ETH won't give you the type of ROI those 'untrusted' altcoins will give if they were to later succeed and not disappear. I guess this is the reason traders take the risk investing in them.
FOMO isn't that bad simply because it can leads you to be in profits sometimes but not every time so its more like how early you enter into the ship and exit at the perfect time before it gets drowned.But there is no other reasons to call it as good, and even you are manipulated so don't become someone who make decision for other's profits.
Honestly, I don't know which is better; FOMO or FUD? But I think while FUD saves you from incurring losses but preventing you from making profits and a good ROI, FOMO gives you that anxiety to buy on impulse and possibly record losses and regrets. You know, when YFI got to over $30,000 traders chased the price up to $40,000+ because of FOMO. Now YFI is trading below $14,000. Those who bought ATH are still licking their wounds from that action. Yje same thing happened to those who bought Bitcoin at ATH in 2017. I rather fall a victim of FUD than FOMO.