This topic gets back to the whole debate of whether TPG (Third Party Grader) holders are safe, or at least safer than long-term exposure to air. Air has the habit of turning paper notes yellow in the short-term, brittle in the long-term. But TPG holders could contain chemicals that slowly leach out and cause a very different kind of harm to notes.
I don't think punching holes in TPG holders is an effective solution toward possible long-term harm reduction. There is very little air inside the holder, holes or no holes, and circulation is going to be very slow. The notes inside the holders are always going to be in direct contact with the clear plastic of the holders, so there does not need to be a gaseous phase that acts as an intermediary in the transfer of contaminants from the holders to the notes. If chemicals in the plastic preferentially leach into a gaseous phase compared to direct transfer between solid plastic and paper note, then punching holes to allow circulation of air into the holders could actually increase exposure of the notes to volatiles rather than decrease exposure. In other words, leaving the holder closed with almost no air inside it might be safer if the holder releases volatiles to the outside, meaning the air around the holder. But, no matter what scenario is true, the long-term safety of a note in contact with plastic that is not perfectly stable is very much in doubt. I have no idea if your concern for the integrity of your notes extends only as far as your tenure as owner lasts, meaning that you only care that your notes remain safe as long as you own them, and once you die or you sell the notes, you could not care less if the notes turn to dust for the next owners.
As far as I am aware, we still do not have closure on the topic of whether BCS company holders are truly safe. I did the Beilstein flame test on them a couple of years ago and they failed. The chemical analyses that were done subsequently by BCS rep Steven Bell seemed to indicate that BCS holders are PVC-free, though the results were not compelling enough at least to me to suggest that the holders are 100% safe. Personally, the plastic used by BCS feels and looks cheap, and I would never consider putting notes inside these holders for long-term storage (i.e., decades).
PMG holders passed the flame test, and they seem to be of a higher quality.
As far as I know, PCGS holders are not perfectly sealed. Didn't someone say they are essentially open at the top end with only the PCGS label covering the opening?
If you are determined to create an opening in any or all of your TPG holders, I would recommend cutting off a bottom corner rather than doing something more visually drastic like using an awl or a hand-punch tool or spindle. Do you remember decades ago when music was sold as vinyl LPs inside square cardboard covers? Some covers had a corner removed to indicate that the record was a "cut-out". That's the general idea I am trying to convey, though, obviously, I would suggest a much more conservative cut if you are going to nip off the corner of a TPG holder. But I still recommend doing absolutely nothing. Any kind of change done to a holder could invalidate the grade assessment, or at least put doubt in to the minds of potential buyers.
