Phase Crossover Frequency at 0 deg or -180 deg
What causes instability problems is positive feedback, i. e., feedback with the same (or close to the same) phase as the input. In other words, 0˚ for the loop gain.

So where does the 180˚ come from?

Typically, it's assumed that the feedback signal is subtracted from the input signal to create the error signal. The negative sign implicit in the subtraction represents a phase rotation of 180˚. So, if you're looking at the phase shift of the loop gain without taking the subtraction into account, then you want to avoid phase shifts close to 180˚ instead.