Choose the option that is cheapest for you. If you lose more than 10% of your income because of losses, use the bank. If it's less than 10%, no-bank.
The maximum amount of fights you can lose in 2 hours, is 20 fights. If you make more than 3 million/hr, it's advised to no-bank, because the maximum amount you can lose is 600K/2hr and that is 10% of 3 million.
It's a different situation in battle. When you lose a lot in battle, you might want to use the bank for those 3 days.