What are some rules of thumb for leaving extra wire within an electrical panel (if any) and routing the extra wire (e.g. create a single loop from the extra wire) to maintain balance between considering potential future wiring needs and also keeping the panel from turning into a complete rats' nest. I am looking for recommendations on additional wiring methods for the wire that feeds the panel as well as the wire that feeds the branch circuits.
Also, with the answers, I am looking for additional reasons why you might need to have extra wire in a circuit panel. For example, I can see leaving some extra wire in the electrical panel for the branch circuits so that you have potential to rework the positions of the circuit breakers in the future without having to run new wire. The only reason I can think of for having extra wire for the main lines feeding the panel is in case I wish to swap out for a different panel in the future which would require the main lugs/breaker in a different spot.
Specifics for My Situation:
I have one 200 A main panel (3/0 AWG wire feeding the panel, spaces for 40 breakers) feeding two other 100 A subpanels (2 AWG feeding these panels) which then feed branch circuits for receptacles (6 AWG to 12 AWG wire feed these circuits). The 200 A panel has spaces for 40 single breakers (~40 inches tall) and the 100 A panels have spaces for 30 single breakers (~30 inches tall) -- I presently plan to have lots of unused breaker space in each panel. I mention this because I figure the size of the panel might also influence the amount of extra wire desired (e.g. in the future, you might want to move a circuit from the top of the panel to the bottom) and the amount of extra wire that the box can accommodate (e.g. if all 40 breakers are used, it doesn't seem like there would be a clean way to leave much extra wire at all).