Article 312 is for panels, and 312.5© just requires cables going into a metal panel be securely attached to the cabinet. Im not following how Article 314 applies to cables entering a panelboard cabinet.
If you are describing this type of installation, keep in mind that the panelboard is installed a cabinet; both defined in NEC Article 100. I believe that Section 312.7 could be applied where excessive lengths of romex jackets are installed into the cabinet.
Article 314 does not cover this question since a cabinet is not a box.
What will you be selling this time Joe? Things must be pretty bad if you have to resort to coming back here…
312.7 Space in Enclosures. Cabinets and cutout boxes shall have sufficient space to accommodate all conductors installed in them without crowding. 
I am very aware that this pertains to Boxes and not cabinets. The intent was to show where the 1/4" came from in someones question which is why it was a cut and paste only and not followed by my own verbiage. Now if you wish to have my verbiage then I will elaborate:
(C) Cables. Where cable is used, each cable shall be secured
to the cabinet, cutout box, or meter socket enclosure.
It is of my opinion that the cable itself must enter into the enclosure as it is required to be secured to the cabinet. As for the 1/4", that is not elaborated but the intent is their. As to how much can be in side the enclosure…I believe their is no limit except to the point it violates Section 312.7. This will be a call the local inspector will have determine beyond that point.
Remember…1/4" is not much…so meeting the intent of Section 312.15©probably makes the question moot on the minimum amount of cable sheathing needed into the enclosure.
Paul, my intent was to point out to Robert the text I felt applied in your posting. It was easier to quote you than to repost.
I’ve been thinking about our discussion and in all likelihood my argument was incorrect. I would say that the section in question is poorly written, at least to me. 
To me is seems the intent of the 314.17 requirements for a 1/4" cable projection in a plastic box are to ensure that knockout tabs that secure the cable are engaged on the cable sheathing, and not the wires. Unlike a metal cable clamp, the plastic knockout tabs typically engage the cable at the very tip on the inside of the box.
If there isn’t at least a little cable sheathing projecting past the tip of the plastic tab to ensure the cable sheathing in engaged, then the wires could be forced out when they are stuffed back into the box possibly damaging the conductor insulation.
I don’t see how even the intent of that requirement applies to panels … at least how panels should be wired, as I have seen some spaghetti bowls … :roll:
JMO & 2-Nickels … 
How is this then…if you install NM Cable in an enclosure in Alexandria, VA…the Code Supervisor (ME) will require that you mechanically connect the NM Cable to the enclosure. I wont get into the weak arguments of 1/4" as i think it would be moot…once you met the requirements of 312.5© anyway. I will defer to Section 90.4 and bid you a fair-well…
The old saying is … “The code is not whats written in the book, its what the inspector says is required. But because an inspector requires something, doesn’t mean thats what written in the book” …