As a general rule, it is always advised to use a GET request for fetching data on the page. This is presumed to be Google’s preferred method of crawling. As an experiment, Google has started to rewrite POST requests to GET. While in most cases it works absolutely fine, in others the content needed by the web server in GET & POST have marked differences. Developers justify using POST with the argument that the data holding capacity is much greater in POST.
Google POSTs are for crawling requests that the page requests automatically. In a way, it mimics a typical user request. Googlebot ensures the performing of POST requests, when it is believed to be safe and appropriate. This ongoing fine tuning of the process will undoubtedly be enhanced with time.