Tracy, I'd say you're spot on. It's a pretty good rule of thumb to wire JBL's with reversed polarity. You seem to have a much better grasp of phase and relative phase than this soundman you're referring to.
The thing that soundman said about one amp's speakers being out of phase with other players in the band is an example of a poor understanding of phase/polarity, and phase coherence. Phase coherence being the least understood of the issues. And the guy, by reversing Tracy's wiring actually did the opposite of what he was probably thinking he wanted.
BUT, it absolutely makes no difference if one guitar player's amp is out of phase with another guitar player's amp. No difference at all. The speakers are not going to be delivering the same exact signal and most definitely are not going to deliver the same signal from the very same point in space since the cabinets are in different locations and could never occupy the same point in space. This soundman need to consider that many of the effects pedals or amp channel switchings that happen in a guitar rig also reverse polarity (absolute phase) in the exact same manner that speaker polarity reversal does. If a guitar player is using a Fender amp, say, like a Twin or Pro, and that guitar player unplugs from the reverb channel and then goes over to the "normal" channel, he has just reversed his polarity since those two amp channels are 180 degrees out of phase with eachother. Is that a problem on stage with the other players? Of course not. If a guitar player has a Stratoblaster with a bypass switch and flips it on or off, he just reversed his polarity. Is that a problem, obviously not. And many amps with overdrive channels will also flip the phase as the drive channel is engaged. Obviously this doesn't suddenly make one's guitar disappear or cancel out anothers'. Relative speaker polarity from one player to another is simply and completely a non-issue.
Phase is an issue among speakers that are mounted in the same speaker cabinet and that are fed the same signal, or among multiple cabinets fed by the same amp rig. If one speaker WITHIN THE SAME RIG is going the opposite direction as another, then we have a problem.
Like Tracy said, no two players could ever play the very exact same note with the exact same amplitude and phase response and harmonic content at exactly the same time down to the microsecond thru the same speaker cab. Those are the conditions where this could even begin to be an issue, and that never would happen in the real world. Two different players with two different rigs that are naturally in two different locations in space will have such radically non-coherent signal sources that they will never cancel each other out as this soundman may have been talking about. Total and utter hogwash to suggest such a thing. I get the theory behind what he may have been thinking, but that conclusion comes from a limited understanding of all the physics at work and the realities of non-coherence and phase, and especially how it relates to guitar signals. Even when the two guitar amps get mic'd and mixed together at the board, perhaps even summed to mono, the two different guitar signals will naturally have plenty of time, phase, pitch, and harmonic differences that they will still both be heard and won't cancel eachother out, no matter which way ones speakers are wired relative to the other.
Now maybe this kind of thinking of the soundman would be better applied to the two kick drums in a two-drummer band where if both drummers happen to nail their kick hits at precisely the same time, having the two mic's in phase with each other would prevent the lowest frequencies from canceling each other out in that brief moment. I get that application of theory in dealing with transients and low frequencies. But the theory and physics behind that thinking fall apart as you go up in frequency and are dealing with guitar signals.
So I'd have to agree with Tracy sticking with the popular convention of wiring JBL's in reverse, just in case they ever are paired up with "normal" speakers. Since basically all other speakers are +Voltage-outward and -Voltage-inward, this rule of thumb helps us avoid situations of phase cancellation in multi-speaker cabinets or multi-cabinet amp rigs.
... and it's just like any other day that's ever been...