It could be different for each investor, most altcoins you can only purchase with bitcoin. If you plan on going immediately USD to BTC to ALTfor every buy, then ALT to BTC to USD for every sell, then watching USD prices makes more sense than watching BTC prices. If not, then you might find it easier to "park" your money in BTC between trades rather than in fiat, and focus on the prices relative to bitcoin (you'd be looking to acquire more BTC with each trade rather than more USD).