So to answer this, the first thing I can remind you is... Gremlins 2, just listen when then are punning about the rules concerning the Mogwai... same here XD.
Now by itself, we find the first depiction of Medusa in the Iliad. Here is the passage with Robert Fagles translation:
Over Athena's shoulders slung her shield, all tassels flaring
terror—Panic mounted high in a crown around it, Hate and Defense
across it, Assault to freeze the blood and right in their midst the
Gorgon's monstrous head, that rippling dragon horror, sign of storming
Agamemnon's shield: Circling the center, ten strong rings of bronze
with twenty disks of glittering tin set in, at the heart a boss of
bulging blue steel and there like a crown the Gorgon's grim mask—the
burning eyes, the stark transfixing horror—and round her strode the
shapes of Rout and Fear.
So at the origin we find her only on a shield. She takes some shape in the Odyssey becoming a servant of Persephone:
The dead came surging round me, [...] and blanching terror gripped
me—panicked now that Queen Persephone might send up from Death some
monstrous head, some Gorgon's staring face!
Notice she is striking fear and paralyze people, even simply on a shield.
Later the main source we have is in the Apollodorus:
Now Medusa alone was mortal; for that reason Perseus was sent to fetch
her head. But the Gorgons had heads twined about with the scales of
dragons, and great tusks like swine's, and brazen hands, and golden
wings, by which they flew; and they turned to stone such as beheld
them. So Perseus stood over them as they slept, and while Athena
guided his hand and he looked with averted gaze on a brazen shield, in
which he beheld the image of the Gorgon, he beheaded her.
Notice here the presence of a
brazen shield in Perseus' hand. As well as the overall monstrous aspect of the Gorgons.
Now you have later Roman author Ovid, and here is the passage in the Metamorphoses:
In the fields and along the paths, here and there, he saw the shapes
of men and animals changed from their natures to hard stone by
Medusa’s gaze. Nevertheless he had himself looked at the dread form of
Medusa reflected in a circular shield of polished bronze that he
carried on his left arm. And while a deep sleep held the snakes and
herself, he struck her head from her neck.
So Homer, Hesiod and Ovid all are saying the same: No one can bear the sight of Medusa. Even Ovid who is clearly mentioning the fact the creatures has been turned to stone by Medusa's gaze still has Perseus approaching her with his shield raised while she is sleeping.
Note also Ovid does not give Medusa's lots of monstrous feature:
They say that Neptune [Poseidon], lord of the seas, violated her in the temple of
Minerva [Athena]. Jupiter’s daughter turned away, and hid her chaste eyes
behind her aegis. So that it might not go unpunished, she changed the
Gorgon’s hair to foul snakes.
See only the hairs here are turned into vicious snakes. Notice also most of the back story we know are from here. When the monstrous aspects are more present in Hesiod (and mostly Ray Harryhaussen good old Clash of the Titans).
Obviously we also have Ray Harryhausen modern interpretation in Clash of the Titans:
An horrible snake like creature with an hideous face. Most notable thing is the actors (not terribly good by the way) almost not move in front of her showing how they are petrified by fear. This can be put side to side with Jason's fight against the 7 headed hydra, the fact they do not really fight the Medusa was obviously a choice perfectly reflecting her monstrous and terrifying appearance.
So you know:
- Medusa has always been associated with shield
- She is an incarnation of Terror (which pretrify you)
- Here gaze turn you into stone (cf. point 2)
- She is so terrifying that even when she is sleeping you cannot stare at her
So factually Perseus does not wear a shield to not see her gaze (she is sleeping) but because even watching her sleeping form is way too much (cf Hesiod and Ovid). That is how terrifying Medusa is.