As a rule of thumb (hah!) I rarely hitch-hike, and never alone, but the San Juans are my exception. As I don't have a car, the roads are aweful for cycling, and public transportation is nearly nonexistant, I rely on hitching quite frequently out here. It is a pretty ingrained and accepted part of the island culture, to the point of having an official hitching post, like a bus stop!
It's been a great way to have conversations with random people that I would otherwise probably never have the chance to talk to as well as learn a lot of history. I've met business-owners, sailors, carpenters, musicians, travelers. Two days in a row I got picked up by two different men who moved out here years ago because they were hired to build staircases from beaches up to people's homes. Who would have thought? I've met residents of surrounding islands such as Waldron and Crane. I met a woman who grew up on the island and moved to L.A. as an adult and became a stripper. She had recently moved back to return to her literal and spiritual roots and become an artist. I've hitchhiked with a carful of children gleefully covered in strawberries and offering me some at the height of the season. I've hitch-hiked with a bucketful of seawater and 2 big dungeness crabs and no one batted an eyelash.
I've gotten to the point where I know some of the people who stop for me, but most the time of course I don't. You're always taking a risk, and in my experience not so much for safety as for social comfort. One time I got in a big fancy SUV with leather interior, carrying my big backpack, and between my hurry to get in and the tall seats... I slid right back out. The man driving looked at me in distain. He was middle-aged, wearing a woven Guatemala-type tunic, long blond hair, and tons of necklaces, driving one of the nicest cars I've seen on the island. I just couldn't connect with him, and our conversation was painfully awkward all the way to Deer Harbor on the other side of the island.
Of course, one golden rule is not hitching when you have a time limit. Expecting a car to stop because you have a schedule is setting yourself up for a bad or tense experience. I leave myself plenty of time, and so far have always had a quick, safe, and interesting ride.