You should never sell your Bitcoin all at once because there is this chance that the Bitcoin keeps rising, you can then make even more money even if you have already sold.
Some people think that you should never sell because they will wait for the future to sell.
If you never sell then you will never make profits from it, if price exceeds then your entry price then its better to sell them off and grab the profits as it will definitely go down in future and that would be the good time to buy it again.