I bought a bunch BTC when it was $17000, then bought a bunch when it is $14000...then the price dropped until 2 days ago. I bought a bunch again when it was 6000ish...haha, now the price went up...so, really, when is it a good time to buy? When it dropped for a while and it rose a little bit, then drop again, then buy?
Of course the best time to buy is when the price is at its cheapest. However, it is not easy, even impossible, to really tell when exactly is that going to happen. So if the dip is already significant, it is better to buy immediately. Never mind if the price continues to go down. After all, it will definitely bounce back after a while. It is much better than not buying at all when the price is low.