Good question. Generally it is recommended to leave the bike in gear in case you need to move quickly and also so you don't need to fumble around when the light changes. Depending on the bike, however, it can also get tiring holding the clutch in for a long time. Don't sweat it if you have to put it neutral to adjust your mirror, scratch your nose, or whatever. Just be aware of what is going on around you and you'll be fine.