After looking in the source code it looks like this parameter is used to set or disable the SCMF_AUTOFIX_TIMING media flag. From what I see in the code, if rtp-autofix-timing is set to true then FreeSWITCH will make it easier for you to communicate with devices that lie about their ptimes or who want to send 30ms ptimes but should really be using 20ms ptimes, etc. On latest git, at line 1337 (yes, really) in switch_core_media.c you'll see it checking this flag (and a few others). Later in that if block you'll see where the famous "Asynchronous PTIME not supported" message comes from.

Perhaps others can chime in on when to set rtp-autofix-timing to false as I've personally never needed to do so.

-MC