I think I've posted on here once before that I often play the middle C# by fingering the low C# then adding the octave key. This creates a C# that has a similar tone to the upper register (the lower part of the upper register, anyway) so it's useful when moving between the upper register and C# because it produces a more consistent sound.
It can also be an easier fingering to move from D down a semitone to C# then back up to D since it only involves moving two fingers rather than 7! Though sometimes it's far more difficult to finger it this way, for example when moving up a semitone from C. It's something to experiment with, anyway.
Cheers,
Mansarde