Is it better practice to specify a relative address in an endpoint for example
<endpoint address="json" ... />
or in the UriTemplates:
[WebGet(UriTemplate="json/....")]
What is better practice here? Does it matter? I know if I decide to change json to js later, I would have to change it in all the UriTemplates, but only one time in the endpoint, but what if I have multiple endpoints. I can't use the same address.
I think this comes down to a question of granularity. An endpoint defines the entire contract you are exposing and a URI template is used to locate/manipulate a specific resource. If you are attempting to vary the output format for the consumer the WCF plumbing handles this for you(by appending the format to the uri, can't recall the specific string).
It is different things. Lets look to each of it.
'EndpointAddress provides a unique network address that a client uses to communicate with a service endpoint'. The endpoint address belongs to the service endpoint, which also contains the binding, contract and behaviors for the endpoint. The endpoint address for a service can be specified either imperatively using code or declaratively through configuration. Defining endpoints in code is usually not practical because the bindings and addresses for a deployed service are typically different from those used while the service is being developed. It is more practical to define service endpoints using configuration. In cases when do you need multiple endpoints defined in your configuration - you can manually instantiate Client using endpoint Name attribute instead of endpoint address.
'WebGetAttribute indicates that a service operation is logically a retrieval operation and that it can be called by the REST programming model'. The WebGetAttribute attribute is applied to a service operation in addition to the OperationContractAttribute and associates the operation with a UriTemplate as well as the HTTP protocol Get verb.
Here is great post about REST / SOAP endpoints for a WCF service where you can see how to configure endpoints for both technologies and how to use WebGetAttribute.