Indeed, as long as you don't sell all of your bitcoin at once, 10% of your savings is a decent amount to get cash in order to buy back if bitcoin price decline. Many people do it everyday, and they call it trading but some people don't want to take the risk to loss, and prefer to save bitcoin for long term but sometimes sell small portion from the wallet. In that way, we could increase the amount of bitcoin as the dip always occurred after high peak.