The best time to buy altcoin is wait for the dip or ICO stage. you can get bonuses if buy on pre ico. if you wait for the dip then you buy on the dip, You don't have to worry about getting a drastically reduced price, and the chances of getting a profit will be higher.
ICO stage or pre-ICO stage also good time but only if the project is having some capability to withstand in th is highly competitive crypto environment. Because, all the ICOs are not going to succeed in coming days. Surely 90% of them is going to fail down. So, choosing the right one is the biggest challenge here, for that filtering out, going along with ICO stages is not a good criteria in my experience.
Buying at dips must be the easiest way to have maximum profits in coming days. Because, you are getting definite profit when buying at dips but again you need to make sure you are buying the right altcoin. Because, many coins may not recover from dips. They will keep on dipping and then get delisted from exchanges. So, simply when you are dealing with the highly reputed coin then most of the problems here will be solved in natural way itself.