While this isn't certainly the most reliable source for information, the MAME page on Wikipedia states that the parent ROM is "the most recent World version of the game". This does sound logical, as one would assume that the last time the game was released somewhere it would also be the most recent version of it.
Now, I don't know if the No-Intro team follows the same P/C standards as the MAME devs do, but if so, there are a couple of things that confuse me.
Let's take "Yoshi's Island DS" as an example. Since the game was last released on Europe, that version would be the parent, which is indeed the case and it makes sense. But for "Zorro - Quest for Justice" (Nintendo DS), which was last released on the USA, the parent is the european version instead. If this is because of the "most recent World version of the game", is Europe supposed to represent the "World" in someway?
Alien Crush for the TurboGrafx-16 was last released on the USA, but the japanese version (which came an year before) is the parent. Why?
This one here is rather subjective, but given on how USA games ran better than their european counterparts (at least when it comes down to home consoles), wouldn't it be preferable to make those the parents rather than the european versions (for some games) even if they represent the most recent version of a game?
That's pretty much all. Sorry if I sound like I'm bashing the team (certainly not

