If you think you might have an emergency( plane, medical, otherwise), declare it. Don’t wait till you know you have an emergency. I remember flying along on a cross country, another general aviation pilot declared an low fuel emergency out in the middle of nowhere. He was below radar contact, but was trying to make closest airport(Santa Rosa, NM). I was 30 miles away, but diverted to be sure he made it, he did, and I doubt if there was any follow up by FAA, but if he hadn’t, someone would of been there(me). I also was able to talk to him when he got below center’s communication. Both he and center was grateful, but I admired the guy for speaking up, many don’t.