For Carrie Waltz, the opening x seconds almost include the key theme, and would be ideal for a .wav. I think that most 19th C. pieces are easy to sample, because they featured repeated verse and refrain. But I could imagine that a more classical piece would be a completely different kettle of fish.
I have to convert FLAC, so for me wave is best, but I know some people work in systems that love FLAC or AIFF. I think .wav is still safest–but actually, for me, mp3 is fine, because a little loss is okay. I just know that wave would be better.