Illinois specifies two methods. The first is the way that motor vehicles are required to do so (use the left hand lane or left turn lane), and the second is to keep to the right, proceed straight through the intersection, and then cross the cross street to achieve a left turn.

So in Illinois, it would be technically illegal to ride up to the front of traffic. I'd hazard a guess that if Oregon doesn't clearly specify a left turn procedure for bikes, then you would be required to follow that for motor vehicles (in which case riding to the front would be illegal).

In practice, I never pull that move. It just seems like a really jerky thing to do. I typically take my place in the line of traffic in the left turn lane.