All my cabs a straight. But, live tone can be misleading from a straight cab. If you are setting up EQ, standing there with your guitar slung, what you hear is not what is going to be projected straight at the front of the stage because you ears are about 3 feet above the projection, more or less - unless you are a midget. If you are on a 4 foot stage, the poor fuckers directly in front are likely to be pounded with the ice picks as your "not so EQ'd" sound is going to be a hell of a lot brighter than you think. You could hear this standing there at your amp, twisting knobs. While this certainly does not warrant changing from straight to slant, it is something to keep in mind while setting up, and the slant I believe will give a better representation of what you want your audience to hear.
Steve