as long as you keep the display hierarchy in mind you can "safely" do this either way (above or below other display elements, as desired)
display has one and only one (at present, though they leave this "open" for future) "stage" (which itself is just a DisplayGroup), accessible via the method display.getCurrentStage()
when Composer is loaded, it's creates its own "stage" (which is a DisplayGroup) as a child of the display stage, accessible via the property composer.stage.
when scenes are created, they each have a "view" property (which is a DisplayGroup) which is a child of the Composer stage, layered in the order of scene creation. "normal" use of Composer will properly handle the layering of any active scene.view's, so as long as all "scene content" is inserted into the scene's view, then inter-scene element layering is also properly handled.
"wrongly layered" elements are typically the result of either
1) failing to insert into ANY group (which then defaults to the display stage, so on top of everything else already there)
2) inserting into the WRONG group (which could place it above/below other elements, depending on where that parent group is)
but also, maybe,..
3) loading Composer "late" (after other groups have created in the display stage, so composer.stage will be on top of all of them) and then failing to "rearrange" the set of display groups as actually desired afterward