My 3mo does it. She holds on with both hands and strokes with her fingers. I know when she's nearly done because she lifts her index finger up and down as if to say "ill be done in a minute" and then does this cute little sigh and then lets go with the drunk face :0)
DS1 used to do it, but DS2 (now 6.5mo) not so much. He likes to hold onto my thumb like a joystick instead. Although he's not averse to bashing my boobs with the flailing arm to make me aware he needs me to change position (we co-sleep) and sometimes he'll try and get to the other boob while feeding off one, but that's Not Allowed (he's already caused me enough pain!) - I can't bear it! He has a tendency to scritch at my boob with his nasty little baby nails that never seem to stop scritching, no matter how often I cut them , rather than palpating or stroking.
But then he doesn't really need the milk to come any faster - we've speculated that the reason he still causes me pain in my L boob (by clamping) is because the milk comes too fast, if anything.