I rotate the left and right tires about every 3-4 runs but if you're normally running mod. motors, 2 is probably more appropriate.
As far as the front tires wearing faster than the rears, that is a normal fact of life with these cars. Like you, I'm not really that concerned with it although I had never really thought of the tire wear affecting caster. Rather, as the tires wear, even during the course of a run if wear is extreme, you create a very slight downward rake that increases the weight on the front end. Or, at least, that's my theory.
As the rear tires wear, you do have to increase the pinion size. As far as how much of an increase, like I've said earlier, I go by feel on gearing rather than using any specific formula.
I've ran tires down to were the foam, glue, and rim meet before if you know what I mean. As long as the car doesn’t excessively bottom out and you can get enough bite, there's no reason not to.
BTW, you can see a pic of the track at SoCal at
http://www.socalrc.com/images/orpano70.jpg . That should give you an idea of what we call “medium” sized in the US so you can see where I’m coming from on gearing.