Adding to what @RShantz said, I'll get on another rider's wheel on occasion and it has happened to me.

If the rider is riding in a controlled manner, not swerving, steady pace, gear looks well maintained then I might latch on but not closer than 1/2 bike length and never without alerting the other rider to my presence. "I'm on your wheel" is typical. It doesn't have to be reckless.


-Tim-