It might be beneficial to study the API’s of other folks to see how they do it. Most of the useful public API’s are published somewhere on the web.
For example, the Overmind project publishes their REST API here. In general, their approach is to return a JSON Dictionary containing the new or modified entity ID and all of its attributes:
Operation HTTP Method URL Query string
-------------------------- ----------- --- ------------
Create node for a specific
provider POST /api/nodes/ provider_id=PROVIDER_ID
HTTP Payload returned
---------------------
JSON dict with id of node created (generated on the server side) and all other
attributes of the node
Twilio’s API is capable of returning XML or JSON. Twilio returns exceptions in the HTTP response body when something goes wrong. In XML, these appear as a <RestException> element within the <TwilioResponse>
In general, I can see returning the object on a PUT or POST as useful, because it will contain any modifications made to the properties of the object (such as default values).