I prefer to divide alignment restrictions by the level of other-worldliness of the critter. Orcs aren't really any stranger flesh than humans or halflings or dwarves, so there isn't any real need to make them evil to the bone.
My goblinoids, on the other hand, came many ages ago from a twisted fey realm, and are strongly inclined towards cruelty, malice and causing terror and pain for it's own sake, as such behavior is 'fun' to them. They aren't just mundane evil (like an orc or a human can be), but have a bit of twisted supernatural evil about them (although not quite to the degree of having the Evil subtype, like devils or demons). Gnomes and Elves, in my setting, are the 'seelie' mirror images of Goblins and Hobgoblins (who totally replace dark elves), and all four of these races have racial hatred against their twisted reflections, replacing any normal racial hatreds, so that Gnomes, for instance, don't care about kobolds at all, but *loathe* Goblins, and the feeling is returned by the Goblins. Bugbears have no mirror twin 'seelie' version, and rumor has it, they killed them all...
It's a kind of 'have my cake and eat it, too' thing. There will be 'okay to kill' critters that aren't just 'diffently shaped people,' and there will be orcs and whatnot, any one of whom might need to be killed because they are a threat, but don't necessarily warrant a campaign of genocide and might be able to be dealt with by treaty or whatever.
