Unwinding the cord is an electrical heating safety issue and has nothing to do with final state of charge the car receives. The surrounding temp and initial battery temp when charging could play a favorable role in increasing what that final state of charge would be but generally is outside your control. On a cool night 65F-70F with the car cooled off, I've noticed the same increase in final SOC ~26-28. A good practice with extension cords as well is to lay them out unwound especially with high loads. Wound extension cords, say 100', have been know to generate enough heat to catch fire when drawing heavy, 20amps, loads. Most chargers has short cords, mine is 18' and I doubt you can wind that heavy cord enough times to really become a safety concern. However, it's a good safety practice and any energy that goes into heating the cord, the charger or your battery is paid for all the same and is ultimately wasted.