You know what they say... trust, but verify.
My opinion regarding trust is a middle ground between you and your BFF, @3Xnocharm
. Trust, but verify. I don't think a couple needs to exchange passwords for everything, but I do think they need to be prepared to unlock the phone and hand it over if their partner asks. If my partner can't do that, it means that he's hiding something from me, and I expect complete honesty and openness in a relationship, so this behavior would be unacceptable, and I would likely end the relationship.
If you and your partner give one another passwords etc, you also have to be able to trust that they aren't going to be constantly snooping. I know the passcode to Real Estate's phone, but that doesn't mean that I've used it to snoop. He has OSFs, as do I. And he has made new OSFs since we have been together, and he has always been completely open with me in his interactions with them, including letting me read all the messages they have exchanged. And we have agreed that if anything about any of these exchanges makes me feel uncomfortable, or feel it threatens our relationship, then I have the right to say, "Your interaction with this person makes me uncomfortable and I feel that it threatens our relationship, so I want it to stop," and he will act accordingly, because our relationship and my feeling of security is more important.
We trust each other implicitly. But that trust had to be earned. He had been cheated on; I don't know that I've ever been cheated on, but in two separate occasions, I was the other woman and I didn't know I was the other woman, so maybe that counts. So we both had/have trust issues, but we're working through them together.
I think trust grows over time, because trust is more important and becomes bigger over time. It's silly and foolish to fully trust someone as soon as you start dating them, because not everyone is trustworthy... you have to get to know them to learn if they are trustworthy, and that takes time. At the same time, at the beginning of a relationship, it's not yet serious and you haven't invested much in that person, so is trust really that important? When you start dating, that is the time to decide if they are trustworthy or not, and you should know whether or not you can trust them before you make a commitment of any kind, and the level of trust should correspond with the level of commitment.