I have two primary areas where I care about the race of an actor being cast:
1. Consistency. If you're casting Denethor as a white guy, Boromir and Faramir should probably be white, or at least biracial, and Boromir and Faramir should "match" in terms of ethnicity. If you're casting for a relatively isolated and insular geography, like the Shire in LOTR or the Two Rivers in WOT, for the most part you should be casting characters and extras as relatively homogenous. It doesn't particularly matter to me what ethnicity you pick, but you should stick with it unless there's a reason not to, which is a great segue into...
2. Plot-important appearances. Obviously, almost any actor is going to deviate somewhat from the source material, and it's not always a big deal. Moiraine in the books is tiny, but Rosamund Pike is pretty tall for a woman at 5'9"; it's a change, but not material to the character. On the flip side, whoever they cast as Sammael (if he's in the show), should be shorter than Rand, because that height difference is integral to the Sammael character in the book.
Similarly, as someone said above, Galadriel has to have hair "golden like the hair of her father and of her foremother Indis, but richer and more radiant, for its gold was touched by some memory of the starlike silver of her mother; and the Eldar said that the light of the Two Trees had been snared in her tresses." If Galadriel is rocking a pink mohawk, she's not Galadriel.