Working with him requires it, for anything where we rely on the information he provides, where the results actually matter.
No, it really doesn't. For the most part, it just involves sharing some info about what we're experimenting with and researching (not
everything, but the stuff that isn't dangerous for him to know--which is stuff we should only be researching in timestop or under privacy spheres anyway), and maybe asking for his insight into certain things.
I mean, we're not going to ask him whether or not trying out X is safe, because chances are he himself has
no idea. He's never seen anyone with our powers before, after all. And he doesn't tell you something is possible when he knows for a fact that it isn't. Him telling Kyouko that it might be possible to turn Oktavia back into Sayaka was deceptive in that, while doing it might be possible, it was virtually impossible that anything Kyouko could try would work.
Trust, but verify is a thing that exists and that we should use whenever we can.
Assuming that someone else is
going to betray you, using any means possible, and is always trying to manipulate you, regardless of much evidence there is to the contrary, is stupid. That doesn't mean you shouldn't be careful. It means that you shouldn't be absurdly paranoid.
We've already seen how far Kyubey is willing to go when presented with a motherlode of a Witch (and Wish). The reason Kyubey came across as so threatening in PMMM is because all of the characters were emotionally fragile, frequently isolated, and they were all facing an overwhelming threat in the form of Walpurgisnacht that they needed every bit of help they could get to survive. Thus, even a small degree of manipulation or lying by omission was sufficient to tip the scales, provided the girls made major mistakes on their own.
Generally, the things we have asked him about and will ask him about involve things that are difficult to impossible to find out otherwise, and things that our own safety and security do not rely on. And he knows that if he burns us once, we're never trusting him again. But we're
never going to trust him far enough to stake our or our friends' safety on it.
Remember what he said? "They simply fail to ask the right questions." In other words, if you're careful about what you ask and ask
enough questions, he'll give you useful information. Even when he refuses to answer, he's pretty obvious about it.
Kyubey is, ultimately, capable of doing far more harm and damage than he is actually doing, and he doesn't do it even when presented with something extremely enticing.
Being wary of him is fine. Being aggressively, insanely paranoid about him is a very foolish mistake, because it drives him to treating you like an obstacle or a problem, rather than someone who is potentially very, very useful.
