Yes. My son is currently on deployment in Afghanistan. He gave me full power of attorney and authorization to open his mail, which he had forwarded here instead of his apartment. I open everything and handle anything I can here. I scan and email or snail mail anything to him that he needs to specifically see. So far, there has been no need to do that.
I open my husbands post all the time, if it looks interesting!! He doesn’t seem to mind, just like I don’t mind if he opens mine. To me if you’ve nothing to hide then why not? I’m not sure what the legal ramifications are but we haven’t signed anything.