In many cases, yes, you can ignore USD. You will want to follow for a while to get used to looking at the charts this way.
So altcoins lag behind when Bitcoin's value increases, therefore the BTC price of an alt will go down. However, when Bitcoins price goes down, the alts go down with it. So you cant be emotionally attached to your altcoins. You want to have stop loses to sell when bitcoin causes them to bleed. Once Bitcoins consolidates, the alts will start to correct in price accordingly. It's kind of the wild west in cryptoland. :)