You are going to find that most leave them intact in the event they are later needed for additional drives or devices with the extra power feeds there. Once you cut the extras off you would need to replace the supply when adding a drive or device. You could roll up one 12v rail and use a plastic tie to secure it out of the way if needed. If you decided on splicing a rail you would certainly want to crimp a plastic insulating cap over the remaining end of the wire to prevent a loose feed from shorting something. Plus if you sold or gave away the case later you would still have an awdward supply remaining. A few plastic slip ties lightly tightened will pull those out of the way.