The 10% figure is just an easily illustrated figure. It all depends on your risk tolerance to begin with, then with your expectations for each issue, etc. I've got current rebalancing thresholds as high as 45-50% right now for core holdings, which just basically means that I want to see the individual holdings run a lot before I take profits, and that means I run the risk of missing a short term selling opportunity where something goes up to, say, "only" 40%. That's because I'm extremely bullish long term on those holdings, but I'm risking a lot of money when I do that. Of course, you're always watching current events, pricing, fundamentals, etc., and when something gets up into nose bleed territory, and things are looking shaky short to intermediate term, you might make a decision to "modify" your thresholds somewhat and scale out a little bit at 40%. I recently did a little of that with ADA, on the previous spike higher.