You'll find there's no right or wrong answer, but really just best and common practices. In some deployments I see traffic shaping rules right on the APs (closest to the edge of the network) but in addition there are typically global traffic shaping rules on the MX as well, so your answer might very well be BOTH depending on what you're trying to accomplish. A common deployment is to set a per-client bandwidth limit on the wireless, and then perhaps more application-specific traffic shaping rules on the MX. Also remember that you can apply traffic shaping if you wish via Group Policy under the Network Wide menu, and then if clients are in that GP, it doesn't matter if they're wired, or wireless, or both wired and wireless at different times, they will be bound by the traffic shaping rules you configure in the GP.
