You’ve been searching for early mint sets, let’s say 1963. You’ve found a nice one. All of the coins still look great. The sealed packaging stayed sealed over these 45+ years.
But, wait. Someone has written on the envelope. Maybe they just wrote “1963 Mint Set” or maybe they wrote their name or “Denver” or something else.
Regardless, the set no longer looks as interesting to you.
Ballpoint ink bonds with paper and becomes difficult to remove without damaging the paper. On the other hand, pencil can be erased, but the imprint of the writing remains on 1963 envelopes.
Yes, some antiques gain value from the writing on them, but that is rare. Generally, antiques with the least damage are the more valuable pieces.
As for mint sets and proof sets, the packaging indicates the type and year of the collectible sets. You really don’t need to re-write the information on the packages.
You will be glad you didn’t when you want to sell those pieces for the most value.
Please don’t write on proof and mint sets with either pen or pencil, and really don’t write on them with felt tips – that’s even worse.