Save USDT is better when bitcoin and altcoin down, but if want to get much profit you should looking for with good coin like BNB, LTC and Doge coin. If you still buy USDT just get little profit and I think is not enough profit earn like buy with other altcoin, I think best way for USDT only when bitcoin and altcoin have lower price, when up back altcoin again.
I think this amount should be around 20-30%. After all, there may always be an opportunity to buy some kind of coin. And keeping everything in altcoins is a very risky step for any investor)