Front and Rear toe in give stabilility, But you trade steering and traction for it. Car setup is all about compromise and tradeoffs. A little of this for a little of that.

Caster gives stability because the contact point is a little behind the pivot. This makes them act a little like front shopping cart wheels, and they tend to stay pointing in the direction you are moving.

If you are running some toe in on the rear, and none or some toe in on the front, and you still have straight line stability issues, you should probably examine your suspension. Check that it is all free to move, and working properly. Make sure your camber and toe settings are identical on both sides of the car. Check that your bearings are all in good shape and free moving. I'm assuming your radio is trimmed properly to give you a straight line at low speeds.

That link i posted earlier is only one page of an awesome read about car setup and handling.