I personally like to keep track in terms of BTC.

Why? Because i like to see how altcoins perform against BTC. BTC is supposed to be a medium of exchange and a store of value, and fiat is definitely not a store of value as far as i am concerned. I'm quite happy to do my accounting in BTC.

This makes a lot of sense for me since i keep a lot of my savings in BTC anyways and my primary goal of investing in alts is to outperform my BTC investments.