Stablecoins are basically Cryptocurrencies but they are called stablecoins for a reason, this means that no matter how volatile the market gets, they would maintain a constant value depending on the currency that they are pegged with, like you said there was a time that Tether (USDT) traded below $1 even though it was tied to the USD, but just too and check it's current rate now and you would see that it is maintaining the price of the currency that it's pegged with, so yes, Stablecoins are a great store of blue if you want the dollar value of your Cryptocurrencies to remain the same.