But the value of altcoin depending to BTC also isn't stable, so the total number of btc wagered for all coins will also have some fluctation, that's just something we have to face as a multi-coin casino.
As for the default prefered currency, most people play btc, so it wouldn't make much sense to show them their btc balance in btc by default, usd is better here. For front page, I would show both, cause yeah, were a crypto-casino, so the total number of btc wagered is surely interested, but the number converted to usd is also something we want to provide to users