Metagaming: Public, Private, or not at all?

[3V4JKZ2]

What are people's opinions on Metagaming here? One player just sent me a message that warned me that the general etiquette was to only provide meta-gamey information via private message, not the discussion thread for the game. Other forums I've been following with Dom3 have had public metagaming, which sometimes inconsistently bugs players in those games.

Is this something that should be clarified by game rules by game creator? Is it more permissible to do the metagaming in private messages, where the person it is directed against doesn't have a chance to respond, than in public, where that person can reply with some of their own material?

For clarification, by metagaming, I mean comments like "Pay attention to military charts at this point", or open diplomacy negotiations, or open calling of NAP-3's, or open "hahaha, I won that fight, maybe you should leave me alone" stuff, or "Hey, playerX has all his armies near me, anyone that wants to snipe at his borders might find this a good time to do it" (especially if you haven't scouted the other side of your neighbours in the early game).

Anything in the OOC thread that would reveal information, basically.

Also, this obviously doesn't apply in any anonymous or no-diplomacy game.

So yeah, what's the communities opinion on this sort of thing?