I think it's too simplified to just say "scores have to be transposed". That depends on several circumstances.
But let's go through things to consider in order of importance:
1. Whatever you do, concert pitch or transposed, the first and most important thing is to clearly write it on top of the first score page. A conductor should be easily able to deal with both concert pitch or transposed scores. What a conductor can not be supposed to silently accept however is having to spend half a hour trying to figure out whether the score is in concert pitch or transposed. In doubt, more information is preferable to less information, when it comes to such central things.
2. Ask the conductor. If you have a conductor who is willing to perform the piece, ask what she or he prefers. There are many different preferences, depending on the style of music, the country you're living in, etc.
3. If you don't have a planned performance for your piece and thus can't ask the conductor, decide for yourself, but optimally (if you're writing the piece on a computer) lay it out in such a way that you can still change it around if needed.
Personally, I write all my scores in concert pitch, with the exception of arrangements, which are often kept transposed. I do this after discussing the matter with several conductors, many of which affirmed that they preferred transposed scores for tonal music, whereas concert pitch was preferred for music without a clear tonal centre.
This has, of course, some reasons: In traditional, tonal music it is generally very easy to transpose voices mentally to get an idea of how the whole piece sounds. If a horn has a written "C" in a Haydn symphony, that will in most cases mean: "tonic!" There is thus a very direct correlation between harmonic function and the notes you see written in the transposed score. Furthermore, it allows you to see the instrumental ranges more clearly, and spot which notes are going to cause more problems due to high/low pitch etc.
In a piece that has nothing to do with common practice tonality, the situation changes somewhat, and even more so the more it moves away from equal temperament and traditional instrumental techniques and notation. In a highly complex Ferneyhough score, no conductor will actually follow an individual voice precisely when conducting, since there's simply too much information to take in. Instead, he will focus on larger things. Keeping everyone together. Watching the dynamic balance and overall timbre, etc. Additionally, you might have many instruments playing in unusual ranges. And microtonal harmonies. And so on. Transposing -that- stuff on the fly for several instruments at once is something no conductor is likely to enjoy. Here, the conductor will generally want to see as quickly as possible the overall density in the various registers, for which a concert pitch score is usually the best choice.
But as I said, in the end it comes down to individual preference on the conductor's part, as well as to local customs.
(P.S. Take note however. Even if you write a concert pitch score, instruments that play in a different octave than they sound are still transposed. A contrabass will still be written an octave "too high", and a piccolo an octave "too low", etc. It is generally advised to write this, too, at the top of your score if you write in concert pitch.)
P.S. Hi guys!