just a couple notes...
it is true that any penetration in the shielding will more-or-less make the shielding ineffective.
it is also true that as often as not you don't really need shielding...
Ferrous shields are intended to block electrical fields, they have very little effect on magnetic fields.
Magnetic fields are best defeated with twisted wire and a well designed input circuit.
So if you keep the twist rate uniform, and reasonably tight, between the rear panel and the jacks you should be ok for power line noise. If you have a real RF problem you will need to look at shielding.
All of this is, of course grossly over-simplified, as proper grounding and power distribution plays as important a role.
Bottom line = "it depends"... I hate it when that happens!