While stretching does have some benefits, it does not appear to affect symmetry.
Abdominals tend to be a somewhat difficult muscle group to develop for most people. Part of the reason is that abs seem to be one of those muscle groups that tend to be more affected by nutrition and compensation. By compensation, I mean that we all have a dominant “side” of the body. We tend to work one side more than the other. While there's no evidence that stretching will affect symmetry, there is value in working the abdominals equally.
Again, there's no evidence that stretching will “slim” the legs. In fact, if your workout induces hypertrophy (muscle gain), stretching will not affect the increase in muscle mass. And, while you can't spot reduce any particular body part, your best approach is to follow a consistent nutritional plan that provides enough calories to support your training goals.
There is some initial evidence that stretching does aide in reducing soreness. A study in the Journal of Athletic Training asked the question:
Among physically active individuals, does stretching before and after
exercise affect muscle soreness and risk of injury?
The study looked at the results of several other stretching studies to identify the potential benefits of stretching. In its conclusion, the study indicated:
“The data on stretching and muscle soreness indicate that, on average,
individuals will observe a reduction in soreness of less than 2 mm on
a 100-mm scale during the 72 hours after exercise.“
However, the study questioned the significance of the the findings and went on further to say in its conclusion:
“In summary, the results of this review do not support the role of
pre-exercise or postexercise stretching as an intervention addressing
postexercise muscle soreness.“
It concluded that more significant research needs to be done to further quantify any benefit from stretching. In my opinion, since there does not appear to be any detrimental effect from stretching after a workout, I'll continue to stretch as part of my post training sessions.
Lastly, if you're interested in learning more about asymmetry, I'd suggest you take a look at Anatomy And Physiology: Human Body Becomes Asymmetrical After Embryo Develops Differences
, a short Ted Talk that does a nice job explaining asymmetry.