I understand the idea of the "bottom line" post a little distinctly. In my understanding it doesn't address the process of arguing (i.e. constructing verbal expressions capable of persuading others). Building an effective argument needs knowing the goal as a prerequisite, obviously. But the situation is different in the private, deciding what to believe. Quite commonly one selects one's belief on a totally inadequate basis (affect heuristics, political sympathies) and then reinforces this belief by arguments constructed with the belief in mind. This is what the post was warning against.
In the analogy with mathematical proofs: if a mathematician is reasonably certain that a theorem holds, he can go and try to find a proof. The proof is an argument presented to the public (here, other mathematicians) and should be clear, elegant and polished. But before that the mathematician must decide which theorem he should try to prove, and it would be a mistake to skip this phase, just formulate a "random" theorem and directly jump to the phase of constructing a proof. In mathematics it would be hard to succeed this way since to decide whether a proof is correct or not is relatively easy and straightforward, but outside mathematics the bottom-line approach is usually feasible and costly.
Your division of reasoning into three steps (guessing conclusion, justifying it, checking the justification) may be inevitable for small irreducible ideas where you can go through the whole process in few minutes. But most arguments are about complex hypotheses whose justification could be (and usually is) reduced to a chain of elementary inductive steps. For such hypotheses it is certainly feasible (psychologically or otherwise) to arrive at them gradually - guessing and rationalising the irreducible bits which can be easily checked, but not the hypothesis as a whole.
Quite commonly one selects one's belief on a totally inadequate basis (affect heuristics, political sympathies) and then reinforces this belief by arguments constructed with the belief in mind. This is what the post was warning against.
That's an application of the post's argument, true. But as gRR notes, the literal meaning of the post discusses how we judge information presented to us by other people, which we receive complete with arguments and conclusions.
Once an argument is given in favor of a belief, and that argument has no logical faults, we must...
In the spirit of contrarianism, I'd like to argue against The Bottom Line.
As I understand the post, its idea is that a rationalist should never "start with a bottom line and then fill out the arguments".
It sounds neat, but I think it is not psychologically feasible. I find that whenever I actually argue, I always have the conclusion already written. Without it, it is impossible to have any direction, and an argument without any direction does not go anywhere.
What actually happens is:
It is at the point 3 that the biases really struck. Motivated Stopping makes me stop checking too early, and Motivated Continuation makes me look for better arguments when defective ones are found for the conclusion I seek, but not for alternatives, resulting in Straw Men.