If you go looking for something, you will find it. Chances are, you won't like what you're looking for so sometimes, it's best to leave things alone. Let situations unfold on their own. Honesty is big in relationships, as it should be. Some couples take that to extremes by giving their partner their SSN, password, bank account information, etc. In a perfect world, you can trust your significant other with that information, but you don't know if they're going to use your information against you. It's best not to reveal sensitive information until you're certain (s)he is thorough. The reason being is so you can have time to observe some things about them; see how they get down before you go sharing your government name. Some people feel if you withhold from your mate, you have something to hide. Whatever happened to personal space?
Some partners think if you withhold, you have something to hide. What does your spouse need with your SSN, your passcode to your smartphone, etc? Nothing. The only reason ANYONE would want the password to their mate's phone is to snoop; that's it. If you can't have an iota of privacy with your significant other, then they're not someone you should be with. Some people are so open that they give their (wo)man the password to their phone, in an effort to prove that "I have nothing to hide". If that's what (s)he wants, that's on them. That's extra ammunition for them to use against you if things should go south. Your (wo)man doesn't need to know everything about you. If you have to search your mate's phone for incriminating evidence, the relationship is doomed. You don't trust them to do right by you, so there's no reason for continue staying.
I don't have time to go through my woman's phone. If I THINK she's cheating, she's out the door (and vice versa). Why waste time trying to look for something that you know you won't like when you find it? You're setting yourself up for hurt when you go looking for incriminating evidence.