Practically, it's quite common to not immediately know the divisor. In cases where the divisor is initially unknown but takes an imperceptible amount of time to compute it's better to render 0%. Otherwise you might get a flash of a full progress bar (for example) while the divisor is determined.
Of course, it's context dependent. As others mention, your code might be full of stuff like X / (divisor || 1).
Of course, it's context dependent. As others mention, your code might be full of stuff like X / (divisor || 1).