I do not always follow this rule. It actually depends on time and  market. I keep buying altcoins for short time holdings like 4-6 months . And If i see that i am getting profit from altcoins than i buy bitcoin for long time. But in some cases I sell Bitcoin if the price goes up too much. And for some times i buy altcoins for long time, and hold it up for the price increase. Then i sell them and buy again btc..