If it's safe to do so, and I definitely won't get in the way of anyone coming the other way I'll almost always go to the front. I tend to get away quicker than most and keep up a relatively quick pace on my commutes. However, if there's someone I think looks like they may get away faster (and crucially, keep that pace up), then I might hang back until they either sprint away from me, or I have to overtake, whichever applies. Also, if i'm tired, ill, or otherwise not on form I may queue instead though if I don't judge myself to capable of keeping up a quick enough pace to stay out of everyone else's way.
I do find it irritating when people go to the front and then dawdle away when the lights turn green, forcing everyone else to move out into traffic to overtake them again, and again, and again. I'd never pull anyone up on it though... I don't think it's often deliberate. Not everyone thinks about these things.