As the price doesn't stop dropping I think it's better to wait a bit more to buy Bitcoins, so we can enjoy maximum profit as possible.
Thinking on long term profit we can buy anytime, but if we can increase our earnings at least on medium term, it's better to wait a bit more. Just don't forget to check CoinMarketcap often.
Buying can be done at any time, but the main point is that you should buy in smaller increments. If you time it right, you can buy all the way down.
The price never goes down in a straight line. It goes down, then gets bought up by those that were waiting to start buying, which repeats itself all the way till where the eventual bottom is.
This will allow people to increase their coin holdings while the price is heading down. I managed to gain almost 7% today by doing so. Right now it's waiting for another dive below $1900.