In real life and in most racing games, you probably want a bit of toe in at both ends.
During a corner, when at the limits, the optimal slip angle for the inside tires is less than the outside tires, so you need toe in at the front and back.
When not at the limits of cornering, but at the limits of acceleration, toe in at the rear of a rear wheel drive car helps stabilize the car. If the car gets a bit sideways due to slippage, the "outside" tire points more inwards than the inside tire, relative to the direction of travel, and you get a bit of self correction effect to steer the rear end back behind the front end.
Front end stablity is mostly related to caster, although most passenger cars use a small amout of toe in.
For racing games, depending on tire wear model, tire heat model, and the amount of grip decrease once the slip angle of maximum force is reached, how lateral and longitundal forces are dealt with in a particular racing game, using toe out can help stablize a racing game car under heavy braking. This was a common exploit used in Grand Prix Legends. What happens is that the tires are at the limits due to braking, so the critical slip angles are very small, if the front end starts to slide outwards, the "outwards" tire gets a higher slip angle and less grip, and the increased grip on the "inwards" tire tends to steer the car back. On some cars in GPL, such as the Lotus, and with braking bias set to a narrow region 52% to 54%, you could end up with a "reverse steering" effect under heavy braking:
http://rcgldr.net/gpl/gplrs.wmv
