It's almost always about what's shortest:
- If it's pronounceable and one syllable, I pronounce it: ORD is ord, RAK is rak, MEL is mel. Except as with most anyone who grew up in Southern California, LAX is L-A-X and EZE is Eazy-E.
- I might invent a pronunciation of the code: wise (YYZ), ackle (AKL), silk (SLC).
- Some letters roll off the tongue: RDU, DME, PVG.
- Short city or airport names include Hong Kong, Heathrow, Reagan.
- B@astardizations of any of the above also go, e.g. bank (BKK), casey (MCI), milexpa (MXP)