My rule of thumb is if you don't take profits, there may be no profit, in the cryptocurrency world. I do hold long positions on Bitcoin and Ethereum, but mainly to buy altcoins when I see them in a good dip. I also take profits in Bitcoin and Ethereum, so it makes it easy to stay long on them. The one thing you have to make sure of is your not selling off your profits in BTC and ETH at a loss when the price dips. I find when BTC is running up in price, it is a good time to buy alt's for short-term investing. That's just my take on the topic, everyone has different trading strategies.