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.
True, they are already tested through time therefore you can trust them.
ETH for example which has dumped below $100 many times already, but it is still recovering, so if you had that timing you will definitely be profitable every time it bounces back.
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.
As what I know, majority of the coins are scam or some has less potential, so if they dump, there's a huge possibility that they will not anymore recover, so it's nice to stay away from them if you can't take the risk, only high risk takers can do that, or they call themselves as a gambler because they are willing to take that high risk, however, if that will succeed, that's big money in return, so really up to you as there are still a lot of good coins with huge potential even if it's new.