Handling Medieval/Ancient Coins: Am I Being Overly Paranoid?

No. As paralyse has already said, they've already survived a thousand years or more without your help, there's really very little you can do to accidentally damage them by casual handling.

As a general rule, there isn't really any such thing as a "mint state" ancient or mediaeval coin - XF is about as good as they usually come. And there's no such thing as a coin that's been sitting around in coin collections since the time of Jesus - coin collecting simply isn't that old. Even the Vatican's coin collection isn't older than the 1500s. All of the Ancient and Dark Age coins that are still around today have been dug up out of the ground, and cleaned. So since they've all already been cleaned, there isn't quite the same stigma attached to cleaning an ancient coin as there is for modern coins. Obviously, you can still do a "bad cleaning job" that makes a coin look ugly and unwanted, but you can do a "good cleaning job" on an ancient coin that would be rejected for a modern coin.

There are a few things to be cautious of, however. For copper and bronze coins, bronze disease can be a killer, A glossy dark green patina on a 2500 year old bronze coin is perfectly normal, but if it's pale green and chalky or powdery looking, it should be treated.

In terms of what paralyse also said: one of the physical properties of silver is that it becomes more brittle with time - a process known as crystallization. A coin that would bend or bounce if abused when freshly minted, would simply snap in half if the same abuse were done to it 1000 years later. Mediaeval silver coins are particularly vulnerable, as they are old enough for crystallization to occur and are usually very thin. I have an ancient silver Greek coin that I personally snapped in half by accident, just by me trying to squeeze it back into a tight-fitting coin flip.

Most of us ancients collectors eschew the TPGs, or any other coin holder that seals up and entombs a coin. We prefer to actually handle our coins, and we can take this attitude because our coins are, for the most part, less fragile than their modern MS counterparts.

Holding them won't cause damage, dropping them can. Years ago I dropped my Tiberius silver denarius on my kitchen hardwood floor. The coin was crystalized inside and brittle. It chipped hitting the floor. I glued it but its value dropped a lot when I dropped it. I paid $175 for it in 1987, don't know what it's worth now. I bought another one later, good VF.

This is a popular type, there are fakes out there, buy from reputable dealers.

Most hammered coins will be fine handled by the edges. You don't want to risk putting a fingerprint on whatever flat surfaces there are. Fingerprints can contain damaging chemicals and you wouldn't think of touching a milled coin.

You can't cause corrosion in silver, gold or even billon, but bronze needs more care. Yes, it survived 2000 years before now but it was buried in the same position covered in encrustations for almost all of that time. Now someone's dug it up, cleaned it and exposed it to the air. Your fingers will add chemicals that can trigger corrosion. So unless it has a good patina, I'd wear cotton gloves.