I have been playing Minecraft with the same Forge version for 2 weeks now, and after I browsed some MOD config files, I loaded back into MC only to see the "Warning: Forge Beta" message, yet I haven't changed my Forge at all since I last played, and when I last played I didn't have that message.
I'm using Forge 36.1.0 and no, I don't remember what MOD config files I changed.