Basically everyone is right. Just pretend you're starting a brand-new ball of yarn.
I'm a little different from the others in how I join yarn ends... I'm what they call a Knotter: tie the two ends together. Then weave both ends into your project.
My reasoning? Better safe than sorry. I've never had a knot give way, no matter how heavily-used the item is. And a scarf gets a lot of tugging and pulling. So do socks. The center back of a sweater? This may be one example where I'd be a tad more comfortable not-Knotting.
But I still opt for better-safe-than-sorry.
There are a lot of other folks who hate the idea of a lumpy knot in the middle of their item. So going knot-less seems to work okay for them.
It's basically a matter of personal preference, I guess.
Hope this helps,