It can affect your "average speed": for every second spent at X-mph below your average speed, you have to spend a second X-mph above your average speed to maintain the average. If you zoom up to the light (above your avg speed), then stop, your "average speed" will be higher (every computer I've seen stops counting when the speed drops to 0-mph). You could also roll up to the light (at X-mph below average), and then when it turns green, bump up your speed to X-mph above average, and then settle back down to your average to maintain it.

However, this is only the average speed when you're moving. To more accurately calculate your average speed over the whole ride, you have to take the distance you traveled and average it out over the total time of your ride. For this "true average speed", it will make no difference (or very little difference) whether you stop at the light, or roll in and then take off again when it turns green.