I suspect that it is not strictly so. You can trade many altcoins such as ETH, LTC, DASH, ZEC, etc directly for fiat, in any direction, therefore you can't say that the price calculations of altcoins necessarily occur through bitcoin, at least not in respect to major altcoins. The price dependence between bitcoin and the rest of the gang is evident but it is definitely not because people price altcoins in BTC. The price of bitcoin is used as a metric to assess the overall market sentiment and that is why prices of altcoins closely follow the price of bitcoin most of the time.