If you look once, irrespective of what you find, then you don't trust your partner. A relationship without trust cannot survive in the long term.
I’ll quote AsTheWorldTurns:
Marriage is a long hard slog and I don't think that it's fatally defective if one checks up on the other at the occasional low ebbs along the way.
I absolutely agree. A lifetime is a very long time. If I have a sudden, unexpected suspicion with good reason, and I check and find it’s nothing, then no harm done. If you know you would check, then still not believe the evidence you found, then fine. Don’t check. But calling one check of a phone in a long term relationship “monitoring” sounds over the top.
Don’t get me wrong. I wouldn’t stay in a relationship where I was constantly suspicious. And if I felt the urge to check often, that would be a sign the relationship was in trouble. But a one off check is not a problem for me. If it is for you, because of your history, then fine.