I often use Bitcoin when trading with altcoins, I have also used USD a few times but I find it does not seem right to me so Bitcoin is still my favorite choice. From the beginning of trade with altcoin, I used Bitcoin. The main reason is that Bitcoin is quite high value, not only that it is predicted to increase many times more in the future. We trade altcoins with BTC, we will earn BTC and if we store, wait for the price increase will be more profitable