I would do it this way:
myBounds = app.selection[0].geometricBounds;
myBounds[3] = myBounds[3] + 10;
app.selection[0].geometricBounds = myBounds;
The point is that because the property is an array, you must set it with an array -- if you try to manipulate a member of the array, what happens is that ExtendScript creates a temporary version of the array, manipulates that and it is left hanging out there with no effect.
Dave