It doesn't really matter, as these are simply transliterations from the Cyrillic alphabet, which I believe has 32 characters as opposed to our 26. There are letters in Cyrillic that we just don't have, so when these are transliterated, they often appear in two or more forms. Rachmaninoff's name ends in the Russian letter B, which is transliterated as V or FF, depending on where it comes in a word. If a word begins with B, it is always transliterated as V, but if it comes at the end, it's often transliterated as FF because that's probably closer to how it sounds in spoken Russian. The Russian letter X (which is the equivalent in Russian to chi in Greek) is sometimes transliterated as "ch" (as in RaCHmaninoff) or as "kh" (as in KHabarovsk).
Tchaikovsky, Rachmaninoff, and Scriabin are most often seen. Also remember that transliteration will be affected somewhat by what language is doing the transliterating. I've seen "Skrjabin" on Henle editions, which is the common German transliteration of his name.