Usually.

Though in Eberron setting, there were "pure-blood" half-elves and half-orcs, because Eberron itself as a plane of existence is only a thousand years old, and it stole lifeforms from around the multiverse to populate it, there's proud halfs who claim their blood has never been tainted on that plane by inbreeding, and they actually get rewarded for it (mongrel halves can't get Dragonmarks, biologically impossible to get the superpower birthmarks, but pureblood ones can).