This document explains the use of HTTP GET and POST.
I think it is an authoritative source.
The summary is (section 1.3 of the document):
- Use GET if the interaction is more like a question (i.e., it is a safe operation such as a query, read operation, or lookup).
- Use POST if:
- The interaction is more like an order, or
- The interaction changes the state of the resource in a way that the user would perceive (e.g., a subscription to a service), or
- The user be held accountable for the results of the interaction.
