@Teal:
well said. I totally agree.
If you can't backstab in a critical way that moves you up several positions in the game, then it's not worth it - this is not a real backstab but just a breach of trust and treaty. These small stabs are remembered and usually give a negative image.
On the other hand, a "real", artfully executed backstab, turning the victim within a few turns from a strong empire into a crippled unorganized mass of armies/countries (Dippy) or fleets/planets (SE) or whatever, is a joy to see - even from the receiving end
. These too are remembered but at least from my point of view and my experiences do not necessarily give a negative image to the player, rather the opposite.