At 12" centres thefirstfully out of phase "dead spot" will be at 1000 hz at 90 degrees to the speakers.
For a spread of 490 degrees, 45 degrees either side of centre the lowest frequency affected will be 1500hz.
For a typical 4×10 the lowest frequency affected is nearer to 1000hz assuming a 45 degree either side of straight on useful coverage. That's three times the highest note of a typical 20-fret bass (E three octaves above the lowest note, 320hz). Obviously the harmonics can be affected but the real world impact of side by side speakers on bass is exaggeratedin my view.
For guitar or vocals I can see it's worse, or for PA with bins either side if the stage. But I think room acoustics will overwhelf such effects in practical situations.
I now await Bill telling me I am wro g and no dubt part of the reason will be off axis interference effects will have a modest effect even when the teo signals are not fully out of phase.