How much time do you have to look, decide and then execute a left turn in heavy traffic? Do you feel pushed by the drivers behind you into making that decision quickly? If you cannot "check your six" quickly and confidently, it's time to wait for certainty.
By "check your six" I don't mean look behind you like a fighter pilot. Instead check to your left for a red light runner, on the left diagonal to make sure your path is clear, straight ahead for oncoming traffic that is a hazard or may not stop, the left turn driver coming toward us to make sure they turn properly, diagonally to the right for a red light runner and finally one last check of the lights to make sure we may proceed.
Beware of the drivers who don't believe that a yellow light means stop. These days drivers are as likely to accelerate when they see a green signal turn to yellow as they are to begin to brake. If you are lawfully in the intersection, you may be wise to wait until your light is red, as you still have the right of way to clear the intersection.
If that left turn is just too dangerous, there is a simple solution. Drive one block past the intersection that you wanted to turn left at and now make three right turns instead. What's a little delay compared to arriving at your destination safely? Reference Links