It's safer to diversify with another kind of portfolio, such as mutual funds, gold index, etc rather than investing in altcoin (even if they truly have a negative correlation or high independence from Bitcoin movement). The crypto market is still fragile, a movement in Bitcoin still affects nearly all altcoins so the net positive is relatively small especially if you're not in the bull market.

If you still want to do it, make sure you're not in a downtrend and the overall market is showing strength against Bitcoin, and if Bitcoin starts to shake things out, close your portfolio quickly. dbshck & ryzaadit have a good point about it.