The wing is where the manual says to put the GMU-22 it and there is a dedicated spot and bracket for it. There is some debate about this location for the GMU-22 but most of us haven't had any issues. Likely, folks are just defaulting to this since that's where the other unit goes.
The solid-state GMU-11 it's small enough to put anywhere. There aren't any restrictions beyond what Garmin says in their manual, just make sure it's protected (from the elements) and not near electronics that could interfere with its operation.
Also, note the can-bus cable length limitations pointed out in the Garmin manual.