You had me worried for a moment with "I like to believe". I considered suggesting the litany of Tarsky, where you basically want to believe whatever is true, and not get too attached to beliefs you may not want. I now see it was just a figure of speech.

Now, would I trust people who would behave dishonourably around people they deem dishonourable? Well, probably less. I’d fear they might flag me as dishonourable, and one sidedly turn what could have been a tit-for-tat into an eye-for-an-eye. I’d be cautious for sure, but I’d most likely still give them the benefit of the doubt until they give me concrete cause to write them off.

Whether that’s a good strategy is another question, though. (For instance, Littlefinger did tell Ned Stark not to trust him, and Ned, being the bloody paragon that he is, trusted the sleazy bastard anyway, and lost is head for it.)