I do agree about it. Do not invest all your money in cryptocurrency. You should keep some money in your offline wallet.
You mean fiat? Well thats the same as keeping your money locked up in a bank with nowhere for it to go or grow. Yeah right you something around 5% Per annum interest rate but thats all. No big investment for you then. But then what are you doing here? 
if you are holding a hold then you don't have to use fiat, you can hold assets that you have in the form of bitcoin and some other coins that have volume and have very high demand, don't let you store assets that you have in coins that don't have volume or demand.
You didnt understand. It you sell coins at profit the USDT can help you store some of that in the form of a stable asset. That does not mean that USDT is worth it. The general consensus regarding USDT is bad and people make up a lot of stories regarding it but they fail to see how we can use it to our advantage. Still do your own research before you buy USDT for crypto.