if the mic is angled to the "face" of the cab (the flat front/grill cloth) it is considered off axis, and will sound scooped compared to an on axis mic in the same position. on axis means the mic's diaphragm is perpendicular to the source (i.e. the face of the speaker cab/flat front of the cab). The same holds true for pretty much any microphone with a cardioid or hypercardioid pattern, straight on will sound less scooped than angled. If you want to try to use 2 mics, be careful to watch phase cancellation/comb filtering as it can alter the tone as well...And in some cases make it seem scooped. In some instances, this comb filtering produces satisfying tonal results, so your ears have to decide. The Fredman technique (2 57's side by side one on axis, one off) is popular in metal circles, as the comb filtering produces a nicely carved midrange. But you can do similar things with any 2 mics.
I like just using a single 57 on axis on my cabs.