I like a different philosophy.

I consider Bitcoins a high risk/high reward type of investment.  Every so often, I'll adjust my investments.  I don't want to have a huge percentage of my portfolio in such investments.  So if Bitcoin goes up a bunch, sell some to keep my risk level low and get some profits.  The more I make, the more I can afford to keep in risky investments, so I don't sell as much each time.