is it ever OK to look at you SO phone without letting them know?

today I have seen at least 2 posts about women casually mentioning going through their SO's phone without telling them or their SO ever knowing about it. some of the responses point out that fact, but some of the responses are positive and completely ignore the violation of privacy. I personally think it's never ok. I snooped on my husbands phone when we were dating and it took him a while to trust me again. we're married now and I still get urge but I don't do it. how do yall feel about this? NB: if you and your SO have a mutual agreement about looking at each others phones, please don't litter the comments with "we don't hide anything from each other!" because that is off topic. i have added that as an answer to avoid the poll being thrown. thanks!