You can sell any time that you like. It depends upon how apportioned you are. If you are 100% BTC, then maybe you should sell a bit. If you are only 1% BTC, then maybe it is a decent time to buy some, and figure out a longer term plan rather than asking on the interwebs about what to do about your personal bitcoin allocation situation.
I have 30 btc to play with and some more in cold storage which i wont touch for a long time.
I dont plan to invest more, but rather want to increase my stash with trading. So far this year was quite good with me (us).
Overall, this year was good for going long, but yeah, you can short or sell and buy back at any time.
I just personally don't think that it is a good strategy to sell on the way down, unless you are pretty confident about the continued downward direction.
Accordingly, I personally employ a strategy to sell on the way up and to buy on the way down, unless I have a pretty high level of confidence that the price is going to continue to go in one direction or another, then I may tweak my strategy a little bit in order to take advantage of that anticipated movement.
So, yeah, each of us employs a bit of a different strategy that is likely tailored to our own risk profile, overall finances and views.
For example, I have been selling BTC on the way up, since about $250, and around $400 or so, I tweaked my BTC trading strategy in such a way that I tend to attempt to keep a certain percentage in BTC and a certain percentage in fiat.
Currently, I am little bit under 92% bitcoin and 8% fiat. If the BTC price goes shooting down, let's say $200, I will probably get into about the 94% or 95% bitcoin territory, but if I think that we have kind of bottomed out on our downward test of support, I may employ a bit more fiat and get into a 96% bitcoin territory (I don't want to run out of trading fiat), on the other hand, if BTC prices goes shooting up, let's say to $200, I may get into about 90.5% bitcoin territory, depending on how quickly it goes up...
From time to time, I will tweak my holdings in such a way to try to push them a little more in one direction or another but my percentages do not really change too radically, and if I were to sell 1 or 2% of my BTC holdings at any one time, that would be a pretty big deal for me (more than I usually do at any one time because I tend to play in smaller increments and I don't really gamble too much with my overall holdings. I just attempt to protect my BTC holdings somewhat through what buying and selling that I do from some of the sometimes extremes of bitcoin's volatility).