Why do X-wing configurations missile/guide bombs use symmetric airfoils?
Assymetrical wings on missiles/guided bombs do exist. However the goal of all missiles/guided bombs is to hit or come close to its target, which mean different things for various missiles. This drives their design:

Symmetrical airfoils on missiles are used in order to guarantee uniform turning capability. This enables the missile to immediately turn in either direction, therefore not losing time to first roll the entire missile before turning, therefore making the missile more maneuvrable. This is especially needed for air-to-air missiles like the R73-Wympel or the Diel IRIS-T. Cruise Missiles, on the other hand, can be less maneuverable (ground targets seldomly move), which is why they do not necessarily have symmetrical wings. For example, if you look closely at the image of the Tomahawk missile, you will notice the unsymmetrical wingshape.  Such missiles use the bank-to-turn control strategy. *.

I do not know too much about this, but the main design goals for these again are good turn capabilities (aka, pull a lot of gs), and supersonic performance. Therefore basically the same considerations you have as for any other airfoil with the exception that the requirements are a bit more extreme.


* An interesting mix is the MBDA Meteor. It uses bank-to-turn because it has underbelly intakes. I cannot find the source, but apparently in the endgame the control mode is switched to skid-to-turn as it is more important to hit the target, then to keep the motor running