I wrote a Scale9Bitmap class in AS3 and it requires the dev to set the BitmapData and a Rectangle for the scale9Grid. I'm wondered what is customary for the public get functions of both. Do I return a clone of each to make it free of hanging references, but not as memory-friendly? or do I leave that up to the dev and return the original instances?
Leave it up to the developer. The developer is able to clone it if required. However this is very error prone. Usually you would shoot for immutable state and return a clone. But this is a bad idea with AS3 as you have already mentioned.