Here's a sure way of not having to worry about it. Do a track stand!
Takes practice but it is the most useful tool in a commuters bag. I use it all the time. I hate having to put my foot down so I've learned how to stay upright on the bike at traffic lights. This trick is most useful on left turns when you have to wait for on coming traffic to pass before you can turn. It also prevents wet feet that get slippery on pedals and prevents those nasty spills on ice. Plus it looks really cool. People look twice and think how does he do that :-)
Here is a simplified way of doing it:
1. When you stop find a spot on the road that has a bit of a bump. On most roads use one of the tire tracks
2. Place your front tire on the lip of the bumb and turn it about 15 - 20 degrees towards the right
3. Lean the bike a bit to balance
4. Use your pedal to gently rock back and forth on the lip.
5. Practice, practice, practice. The best place to practice is on a driveway that has a bit of an incline.
6. Have fun.
For a picture of what it looks like. Look under the pic contest thread and find my thread and the picture shows me doing a track stand.