The RPC Spec doesn't make mention of this but it seems like it really would be required. You can't move something if you don't know where you are moving it to.
I'm writing a Java library to access Transmission via the API and just wanted to see if I should enforce the location argument for this method.
Haven't read the specs myself but unless it says something like that the default location will be used if none supplied then it's kinda self evident that it's needed. Personally I would go the safe route and enforce it for now.