It is almost simple like if you are in extreme need of it then obviously go for selling it but if you dont need it too much then keep your all bitcoins with you like time will come in which its price will rise enough to fulfill you desire in which you will have handsome return in shape of profit .so i will suggest that keeping all your bitcoins is the good option.
Longer term you are right but you overlook the fact that many try to to buy the dips and sell the peaks in order to grow their holdings and I guess that's what most are talking about. In my opinion a price of 10k is about in the middle, I expect we might see a price of about 9k again but also that 11k is equally as likely in the same period.
The right time to sell bitcoin or any other asset regardless of form is when the holder is in great need of money. I highly support your this statement. Otherwise, it is best to buy hold on bitcoin. Bitcoin is predicted to rise a lot in coming years. I dont think the same because digital market cannot be predicted with accuracy for that long. However, for coming few months, I see good progress in its value.