What drives toe settings and when would you deviate from factory settings?
 

Is part of the reasoning for toe-in to accomodate a certain amount of deflection in the suspension when fore and aft road forces come in to play ?

If this has something to do with the factory settings, then what about the case where a 911 has things like Chuck's Poly-bronze bushings and monoballs in place of the more compliant rubber bushings? Is there a case to decrease toe at the front and rear? Any opinions?

There is a case or two to deviate. Less toe, less rolling resistance. Also, different handling characteristics. For the average freeway driver, stability is more important than for a racer. Some autox guys will even toe out the front for quicker turn in. A handful on a long straight or the hiway to the event if driven that way. But good stuff on the track.

Yes, toe changes with accelerationg and braking, especially in the rear. Monoballs and bearing-bushings will hold better settings, but there is some built in toe change with height change on the rear. You set your desired toe at the the desired ride height.

One the front, toe changes with ride height change, too. That is called bump steer and can be worked on and made to be better.

Slight toe in is desireable, but I ran zero. I had a lot of negative camber as well, so I can't tell you which made the car darty on the grooved hiway. It seemed to handle very well at our local track at speed. Always went where you pointed it, even saved itself once on a big wiggle and potential tank slapper. I say it saved itself because I sure didn't save it.