The general rule is buy low and sell high but it depends on your expectations and your goals. The price is low relative to where it was a week ago but high relative to 2 weeks ago, then again it's very low compared to just a couple of months ago.
If you believe in bitcoin and are in it for the long term then the decision is much easier because you can be safe in your assumption the price will continue to grow and reach a new ATH later this year. With that in mind it seems like a good time to buy right now.
Really for someone who is holding for very long positions anyway, and there is no knowledge to analyze the chart to know when to buy in, I would rather say, they start buying in margins. We have gone down and we do not know how long we can still go, but the market will always recover, so in that case, buying in percentages is always good enough and at least you have yourself covered even if the market goes down more, or it starts recovering.
Bitcoin should be purchased at the most lowest price that you can enter into. Now would be okay or around $8, if it goes down there.
I would not be surprised if some people may still be looking for something lower than what we already had which even made some long term holders to have missed a mouthwatering offer at $6000 less. No one can predict when we will break out from the downtrend channel, but at least, we will at some point, and the most important thing is to make use of the present moment to stash in more, which I believe every investor must be considering by now anyway.