Well it depends on the coin and the term of the investment, buying a strong coin immediately is good, if the price go down it tend to go up in the long term, but if you buy weak coin then need to be careful because once it drop it could be hard to gain momentum, nobody could predict correctly 100% when is the bottom, but by learning indicators and market situation it could help us to determine where will be the bottom
Indicators only work for coins that are active in the market, no matter how deep you learn to use indicator, it just would not work for some coins, the only coins I have been able to successfully used indicators for which I believed had bottomed already long time ago and now on their bull run is Bitcoin and Ethereum, those are the only coins I trust indicators to work for.
I have so many weak coins in my portfolio that I have tried using indicator for but didnt work and now they look like shitcoin for me already while Some other altcoins we desire to invest in can only be bought with faith, believing that they had already bottomed.