The response to a POST request is not cacheable, due to an underspecification in the HTTP standards. Section 13.4 does not forbid caching of responses to POST requests, but no other part of the HTTP standard explains how the caching of POST requests could be implemented, so we are in a vacuum. No existing caching servers implement the caching of POST requests (although some browsers with more aggressive caching implement their own caching of POST requests). However, this may change if someone does the groundwork of defining the semantics for cache operations on POST requests.
Note that if a Squid accelerator is being used, you should be aware that it accelerates outgoing traffic but does not bundle incoming traffic. Squid is of no benefit at all on POST requests, which could be a problem if the site receives a lot of long POST requests. Using GET instead of POST means that requests can be cached, so the possibility of using GETs should always be considered. However, unlike with POSTs, there are size limits and visibility issues that apply to GETs, so they may not be suitable in every case.