The popular opinion now is buy the coins that are down, and when the market recovers that you'll be in profit. While that saying is correct but that doesn't go for all coins. Don't go buying every coins you see losing value just because you think when the market bounces up, they'll follow. Majority are losing value because people are seeing them for what they truly are which are just a waste of investment and time, also a ticking time bomb waiting to explosed.
I don't follow the rule buy when this coin because it's still cheap I like to see the potential of the market and why it's going down, every coin suffers from uptrend and downtrend you should check what is the reason why the coin is moving that way, not because of the market condition but more so on the opinion and reaction of its investors.