It depends on your strategy for me I don't wait until 20% for me to take profit, most times, some coins hardly make such gain, but taking profits like 5% or 10% many times could be a good idea, coins normally makes such gain many times before it could go as high as 20%, so I believe in taking profits as soon as you see them and re buy when it dips down again.
That's a great idea, it's better to take small profits rather than give up the opportunity to sell right? most people make the same mistake repeatedly because they are too greedy by waiting for the price to rise higher precisely what happens is the price decreases due to certain factors, so use the opportunity that comes as good as possible don't be too greedy