helmet all of the time. I am almost 40, have a kid, and don't want to die.
Most of my scariest situations are when riding trails or paths and coming up on walkers with headphones in, with dogs on long leashes, retractable leashes or off leash. Or groups of walkers that walk 3-4 wide and don't consider anyone else using the path.
You gotta yell early, loud and often. I find they often jump into the direction you say you are coming past.
Last fall I was riding on a single track trail, and I came up on an old man with earphones in, and he was was not hearing me call out, and he was bending a small tree (I have no idea why) and I went to go around him, startled him, he let go of the tree and it hit me in the face (that is when I realized he had been bending the tree) causing my glasses to smash and me to fall off the bike. He started to laugh at me. It took every ounce of my soul not to murder him. I got back on my bike and continued on.