Direction of Current On Resistors
You can choose any polarity for your analysis. Basically, you are choosing a reference direction for the current. The result is relative to that polarity -- so if you get a positive answer, then current is actually flowing in that direction (w.r.t. your initial choice of reference). If you get a negative answer, then 'negative' current is flowing in that direction, or 'positive' current is flowing in the opposite direction.