Ok, this is ridiculous.
Any tactic is valid in diplomacy, including throwing an unwinnable game to Player A to piss off Player B (who is attacking you incessantly), if it allows you to meet your goals in the end.
If your 'goal' is 'solo victory whenever possible', stab away. If it is 'be trusted as a good ally, so those rare stabs are that much more effective when they come', then stick to your alliances.
Generally I stick to my alliances, it doesn't necessarily always work out. That doesn't mean I'm going postal on everyone every time I get stabbed, it means that *in general* I try to develop a rep of a person who keeps his word. This makes other people *much* more likely to work with me against Mr. Stabbypants than hope Mr. Stabbypants won't want into their unsecured rear areas on a whim.
If I *know* you're going to come for me sooner or later, I'm certainly not going to make it *easy* for you, which makes it harder to pound on someone else. Which usually means someone *else* wins, because I'm busy protecting against a stab instead of gaining SCs