You can sell your altcoins when BTC price is high when paired with USD. This means that when you sell your alts, you will get less BTC the amount will be more when paired with USD.
Also, if the price of alts is rising while BTC's price is not then it is a good time to sell the alts. It is often seen that alts price keeps depreciating when the BTC's price increase, hence it is the best time to sell the alts when BTC's price and alt's price increase.