Obviously it makes sense to apply to schools that provide you the best opportunity of a career you are seeking after graduating.
Firms is one component, which gives an idea as to how much presence in a field the school has. Second, you ought to heavily consider location. Nearly all schools have a stronger ability to place within their local proximity, with a reduced ability to place outside of that. So applying to a school in the wrong area with the right firms may result in you having to shift again to get what you want, or involve playing with reduced options.
There is a component that is along the lines of
the best laid plans of mice and men. In this way, you should look for a depth of firms across other fields in which you have some interest and perceive you might be drawn to. That way you have a back up line when you realize you were completely wrong on the ideas you had about going into job X.