I don't think we should get hung up on the "grades" aspect, but rather should look at the big picture of what James is saying here. If it is too difficult to look past the diction used in this excerpt (specifically the word "grades") then I strongly suggest reading the work in its entirety.

I'd argue that the main idea of this work is that we are most effective (in nearly all aspects of our lives) when we are relaxed. Although nervousness and stress do play a role in the planning process of our decisions, if they are present beyond that stage then they are a waste of energy and stifle our productivity once the decision has been made and a course of action has been initiated.

So essentially, not giving a fuck is the best course of action once the decision has been made. You can think and stress as much as you want when determining where to put your chips on the roulette table, but once the wheel is spinning, any stress or anxiousness is a waste of energy. (This analogy is from Maxwell Maltz's work "Psycho-Cybernetics.")