this is a very good point which i usually tell others too but i wouldn't say any percentages though. because whether it is 5% or 30% or even 150% depends on the situation a lot. for instance there are times when in the altcoin market they are pumping some shitcoin up fast and it is obviously going up a lot, getting out in 5% is not a good idea and it doesn't necessarily have to be greediness to wait either. it comes down to analysis of the market and speculation.
additionally you can always both get in and get out incrementally. meaning you buy multiple times at different prices (eg. buy at 1500 sat, 1550, 1600) and dump multiple times at multiple prices (eg. 1600, 1700, 2000 satoshi).