Even in games where I'm running 90-100% science, I'll still usually build banks in my 3-5 best cities, at least past the mid-game. In those cities, it doesn't take that long to build them. On the last turn of research for every tech, I'm going to turn down the science slider, so those 3-5 will more than pay for their upkeep costs, even if they're a loss on every other turn.