there is a very easy way to texture with symmetry.
this tutorial shows how to bake a texture from one object to an object with different topology and different UV’s.
this is how i did it:
-model
-make UV’s. UV’ s dont need to be symmetrical.
-duplicate the model
-delete one half of the object and add a mirror modifier. you can now texture and both sides will have the same texture.
-in the last step you need to follow this tutorial . http://www.screencast.com/users/blenderwho/folders/Jing/media/8999e39f-0a49-464a-b558-e3ac2bcc8c4f
so you have two objects with the same topollgy and different UV’s. you bake from the mirrored model to the original model. and if everythign works you should now have a perfect mirrored texture on the original model . i think this is quecket and easier then other techniques.