A) As for leaving feedback, I understand that it is mainly to be used if 1) you have traded with someone or if 2) you would trust someone to trade, even if you have not traded yet. This second part is more subjective and can be based on his behavior in the forum. 1) is straightforward, for example, you can leave a feedback: "I traded 0,1 BTC for x Eth, trade went smoothly".
Well, it depends: If you traded with someone, does that mean you trust him? If you didn't risk anything, neutral feedback may be the better choice.
But don't you think that leaving feedback because you would hypothetically trust to make a trade with someone (2) is more subjective and may result in people leaving feedback simply because they like the other person, for example?
I've received several positive feedbacks from (by now) DT-members, many of those have nothing to do with trades. Obviously it's subjective, but those feedbacks are a lot harder to "earn" than for instance trading 0.1 BTC for a shitcoin.
seems to me that I will have to rethink and maybe delete or modify some feedbacks I had left.
It never hurts to check if old feedback is still appropriate.
B) As for building the trust list, how do you do it? I mean, I think it's clear what needs to be done, but how often do you add or remove people? Do you do it as it comes? If you see someone leave feedback that catches your attention, you go and look at their trust list and then decide whether to include or exclude them from yours?
I like to say I'm quite conservative: I barely change my Trust list, and most changes were to exclude someone instead of including them.