Laudator Temporis Acti

Homer, Iliad 13.288-291 (Idomeneus speaking to Meriones, tr. Samuel Butler):

If you were struck by a dart or smitten in close combat, it would not be from behind, in your neck nor back, but the weapon would hit you in the chest or belly as you were pressing forward to a place in the front ranks.

εἴ περ γάρ κε βλεῖο πονεύμενος ἠὲ τυπείης
οὐκ ἂν ἐν αὐχέν᾽ ὄπισθε πέσοι βέλος οὐδ᾽ ἐνὶ νώτῳ,
ἀλλά κεν ἢ στέρνων ἢ νηδύος ἀντιάσειε
πρόσσω ἱεμένοιο μετὰ προμάχων ὀαριστύν.

A wound in the back is generally disgraceful, because it means that the man struck was fleeing, and it is shameful to flee in battle. Examples in the Iliad include 5.38-42 (Agamemnon slays Odius), 11.446-448 (Odysseus slays Socus), 16.307-311 (Patroclus slays Areïycus), 17.578-579 (Menelaus slays Podes), and 20.487-489 (Achilles slays Areïthous).

By contrast, a wound in the chest or belly is honorable, because it means that the wounded man was facing his foe and fighting bravely when struck.

Richard Janko, in his commentary on books 13-16 of the Iliad (Cambridge: Cambridge University Press, 1992), cites two fragments of Tyrtaeus as parallels to 13.288-291.

Tyrtaeus 11.17-20 (tr. J.M. Edmonds):

For pleasant it is in dreadful warfare to pierce the midriff of a flying man, and disgraced is the dead that lieth in the dust with a spear-point in his back.

ἀργαλέον γὰρ ὄπισθε μετάφρενόν ἐστι δαΐζειν
  ἀνδρὸς φεύγοντος δηίῳ ἐν πολέμῳ·
αἰσχρὸς δ' ἐστὶ νέκυς κατακείμενος ἐν κονίῃσι
  νῶτον ὄπισθ' αἰχμῇ δουρὸς ἐληλάμενος.

Tyrtaeus 12.23-28 (tr. J.M. Edmonds):

Moreover he that falleth in the van and losest dear life to the glory of his city and his countrymen and his father, with many a frontwise wound through breast and breastplate and through bossy shield, he is bewailed alike by young and old, and lamented with sore regret by all the city.

αὐτὸς δ' ἐν προμάχοισι πεσὼν φίλον ὤλεσε θυμόν,
  ἄστυ τε καὶ λαοὺς καὶ πατέρ' εὐκλείσας,
πολλὰ διὰ στέρνοιο καὶ ἀσπίδος ὀμφαλοέσσης
  καὶ διὰ θώρηκος πρόσθεν ἐληλάμενος.
τὸν δ' ὀλοφύρονται μὲν ὁμῶς νέοι ἠδὲ γέροντες,
  ἀργαλέῳ δὲ πόθῳ πᾶσα κέκηδε πόλις.

Vergil alludes to this same idea at Aeneid 11.55-56 (tr. H. Rushton Fairclough):

Yet not shall thine eyes, Evander, look on one routed with shameful wounds.

at non, Evandre, pudendis / volneribus pulsum aspicies.

"Shameful wounds" would be those inflicted in the back of the body. Turnus with a spear cast dealt Evander's son Pallas a fatal blow in front (Aeneid 10.482-485, tr. Fairclough):

But with quivering stroke the point tears through the centre of the shield, with all its plates of iron, all its plates of brass, all the bull-hide's overlaying folds; then pierces the corslet's barrier and the mighty breast.

  at clipeum, tot ferri terga, tot aeris,
quem pellis totiens obeat circumdata tauri,
vibranti cuspis medium transverberat ictu
loricaeque moras et pectus perforat ingens.