When traffic is light and I can easily merge into the left lane, I usually do. However, when its bumper to bumper or cars are really flying, I usually opt for a chicken left - ride across the street to the opposite corner, then cross the street when the light turns and traffic has stopped. The other thing that helps when making a left, (from a left turn lane or merging into the left lane), is sticking your left arm out to signify a left turn - this letds drivers behind you and approaching you know your intentions.

I don't have any stories of drivers stopping and thanking me for signalling or being a safe and courteous cyclist, but I've had very few run ins with them and I think because I'm relatively predictable when I ride might be a reason why.