What's the proper way to leave extra length of wire going into electrical panel?
The Code requires "neat and workmanlike" work.  A lot of people interpret that as nipping back all extra length of wires so there's the minimum wire clutter possible.  I don't at all.  I leave enough wire so I could move the breaker almost anywhere in the panel, and I use stranded wire so shaping them in clever ways doesn't work. 

One very easy way of leaving extra space is intentionally choosing breaker locations that are farther away than needed, i.e. wire all the left-entering cables onto the right breaker bar, and the right-entering cables onto the left breaker bar.  Remember don't cross the buses with wire, go above or below the buses. 


  110.12 Mechanical Execution of Work. Electrical equipment shall be installed in a neat and workmanlike manner.


That's it.