On entering the main panel, sufficient branch conductor wire must be left in the box in case any rewiring changes are made. Generally just enough wire to run straight to the neutral or ground bus and the hot to the appropriate breaker is not considered sufficient.
I like to leave about 18 inches or so, then simply run the wire up or down anto a long U shape, and then to the bus or breaker, as seen in the picture here. Looks really sweet in my opinion.
