I have. My fiancee and I had trust issues - after losing his son in a custody battle, our relationship went downhill very quickly as we both stopped communicating with each other, too absorbed in our own worlds. I started noticing that he was texting a lot more, and then got suspicious one night when a woman's name I didn't recognize came up on his phone. I usually answered when he was in the shower, etc., because we trusted each other that much. So I answered, and it turned out to be an affair. (I confronted him about it and we have since worked through our issues of communication and entered into a deeper relationship.)
For a good year after that, I would sneak peaks at his texts. At first it was a way to catch him, but eventually it became a positive thing. The more I checked, the more I knew he wasn't cheating because none of the texts were ever suspicious. While I know it's not the sole indicator of affairs, it eased my mind enough that, after a year, I finally stopped checking them.
We still answer each other's phones occasionally, especially if we know the other would take the call (from family, the lawyer, etc.). But we also respect each other's space and I don't venture into his texts anymore. But we both are fine with the other "checking in". We trust each other to not cheat, or to address issues before they get to breaking point. I take it of a sign of our strength that we both trust each other enough to respect our space but also to allow the other to enter that space to erase any doubts. Because, let's face it, we all have doubts.