Hey, after decided to fiddle around in D dorian on real staff paper, I came to a conundrum.

D dorian is in the key of C, correct?

If so, I would leave the key signature a C major. However, at a glance to someone, it would appear the song is in C major, when in fact it isn't. Alternatively, I could notate the key as D minor, and put a sharp next to every B.

So which would be the most common, or even, the most preferred? Putting C would keep the reading simple; but putting Dm would instantly say it's in D dorian due to the sharpened sixths...
you are correct in your theory. However D dorian is still in the key of D as D is the root note. However it is the same scale shape as C major. A good rule of thumb is: In a major scale set of modes, if you play a numbered note's major scale, it is the same as the mode number in the original key major scale. D dorian is the same as C major as C is 2nd to D. A D phrygian is the same as E major as E is third to D and so on. The shape remains, but the root note shifts.
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Wait I screwed up. D major is the same as C dorian. Forget what I just said, I must still be a little tipsy from last night. To answer your question, you would still make the key signature D, however when the dorian D pops up, you would annotate a natural symbol by the F and C as they are sharp in the D key signature but natural in the D dorian. Sorry for the confusion,
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Wait I screwed up. D major is the same as C dorian. Forget what I just said, I must still be a little tipsy from last night. To answer your question, you would still make the key signature D, however when the dorian D pops up, you would annotate a natural symbol by the F and C as they are sharp in the D key signature but natural in the D dorian. Sorry for the confusion,

Technically, C# locrian should be the "same" as D major. I'm not talking about anything related to guitar; just pure music theory and musical notation.

I'm assuming niguitar's response confirms my suspicions that D dorian, E phrygian, et cetera, are in the key of C. So I'm taking it that when writing in modes, for simplicity you should write the key signature as it's actual key regardless of root... yeah, I'll just keep it as that, and write something like "dorian" above the piece, but keep in mind it's like D minor with a raised sixth while writing.
D Dorian is a form of D minor. The key would be D minor, with a natural sign next to every Bb (to make it the B from D Dorian)

No sharp sign, you don't make it a B# (or #6)!!
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Wait I screwed up. D major is the same as C dorian.
C Dorian and D Major are quite different, actually. D Major contains two sharps (F# & C#) and C Dorian has two flats (Bb & Eb).
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There is too much of this ambiguity between modes and scales and proper keys recently. You only ever have major or minor keys.
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