There are always gonna be people that I trust 100% within the game, and I'm happy that in most cases they seem to make up a large portion of the tribes I'm in (<3 to those concerned). Ideally, one's tribe and one's allies should be 100% trustworthy, but seeing as there will always be a limit on the amount of people one has that sort of trust with, there are generally a few people who you don't know that wellwhom you can't trust to the same extent. Often you will need to have faith in them, but that doesn't mean you have to share everything with them
In short, no, I don't think there will ever be a situation in which you can explicitly trust everyone you need to work with straight, but as clur said, a good tribe is one in which everyone can trust each other. A good tribe is a tribe which hasn't had to recruit to the stage where one has to be worried about spies, and a good tribe will work together to such a degree to the point that if there wasn't trust before, there will certainly be trust soon enough.
I kinda contraditced myself a lot there... Perhaps I trust too much in myself and my ability to be clear and concise