The thing about interest is that it isn't instant but adds up. To see a huge change while having 5-6m in bank, you'd need to wait a while - a long while. You get around 15k a day, and for most people this is just pocket change to train a horse or something such. However, if you wait, say, a year without checking it, you'd have around 11.25m instead of 6m. Obviously, 11m isn't a ton with the HI economy now either, but your bank account has almost doubled.
Since I feel like explaining where I got 11.25m from, here's the quick math;
Anyway, the point of that was to say that having 6m doesn't give you much back. I only noticed interest making a difference after about 50m in bank with 120k being produced a day - even more if you have the additional 25% club Investors bonus. If you can buy subs out of game, great! You'll be able to save more efficiently if you put everything you earn into the bank instead of buying another horse or the New Fancy (Expensive) Thing. If you can only buy subs in game, it's definitely still possible to get a nice amount in bank, you'd just need to find other ways to make enough money to stay subbed while saving. Good luck! 
