Depends on everyone else in your group. If everyone else is non-stabbing types and you go and stab them then next time no one will ally with you. If several others are of similar mind and enjoy playing to their full stabbing potential, then that isn't a problem and the non-stabbers need to adjust.

But at the same time, each player has to ask themselves if they are having fun. If you all go full cutthroat on each other but one or two aren't enjoying that style of play, then they may stop playing. So since you have this closed group, you want to make sure the way you are all playing is fun for all.

Hope that is helpful.

IMHO there isn't a 'right' way to play diplomacy in so far as most techniques will meet a technique that will beat it everytime. Perhaps it is better to vary your style to keep everyone guessinga little bit, much like in poker, be difficult to read.

Whilst there isn't a right way, there is a wrong way and that is entering a game with a pre-conceived idea of who you are going to ally with or gang up on. Try and leave any baggage at the door-step of the last game, the problem with meta-gaming is that if it is successful within a group 2 or 3 players will benefit from it but what of the others? They will keep losing and then lose interest in the game and then the meta-gamers have no one to play with.

Keep meta-gaming to the absolute minimum, we all know it goes on particularly in leagues and closed games of friends, but it doesn't add to the experience in the long run.

The right way to play diplomacy is to give me all your scs.

I think the goal should always be to win. If that is not achievable draw.
So I think some back stabbing is going to be required. Everyone should be prepared to face that, and should never leave themselves overly vulnerable to the stab.
I believe that is the way this game should be played. Some people play for points, or join up and go for a tie, I personally don't like that. It makes it difficult for others in the game, if they don't have a (i will never break an alliance) partner.