frankly im not sure snooping makes a huge difference.
my ex knew some of my passwords in the beginning. i think she actually had one or two before we were together, for various reasons. im a huge stickler for privacy, though, and even more so now. as our relationship went on though, it got a little crazy, she was constantly getting into things of mine, and finding something (where there wasnt anything) to start a fight over. even when i would change my passwords, shed find a way, it mystified me. there was the time i found all my usernames and passwords written down in her closet, and that was most definitely not cool. the email attached to my facebook (the existence of which i ignored) she got into while we were together, as well as once she got with my replacement. shes a huge, huge privacy invader. odd thing is, she somewhat practices what she preaches, and leaves all of her things open.
however, from time to time, during the relationship, i would snoop solely to see if she was snooping on me. more than half the time, she had been. but i never found even the slightest bit of evidence she was unfaithful, let alone even flirtatious, with anyone. point is, i now realize, even though i practically lived with this person, she had several contacts i never heard of, and probably a number of guys to, at the very least, flirt with.
if ive learned anything, its that possessive/jealous types, privacy invaders and the like, are often cheaters, or will often cheat themselves. i truly never had the foggiest idea about my ex (there was one single incident i should have known better about but dismissed), yet i was fooled.
i look at privacy this way. if youre not married, privacy should be 100%, absolute. when youre married, the rules change a bit, but privacy and space are still a must. i mean, you share bank accounts, and the like, you may need to exchange a password here and there, etc. its like my mother says. her computer is completely accessible, and anyone could go through it at any time. however, if anyone did, she would change it and prevent it from happening again. the best reason not to snoop, is that youre almost guaranteed to find 'something', whether its "something" or not, and make 'something' out of it.