For me personally buying altcoin and bitcoin when the price goes down is the right decision,
for btc yes since btc is only one but for alts ?you must be specific if what alts because alts have many kinds. you cant say invest in all alts because that will be expensive and that is totally a bad decision to make . you can just say invest in btc and eth because of them are always the perfect combination .
but don't forget to observe and analyze what causes the price of the coin to decrease
why would you bother to do that ? its not already your job to analyze the price fall . its already given that the price fall because somebody sells , thats it .