The last band I was in played solely in C-standard tuning, so that's what I was tuned to for quite a while. I actually tuned my 6-string bass up a half-step from standard tuning instead of tuning it down 2 whole steps as the strings would have nearly fallen off, haha.
Right now I'm usually in E-standard for my project This White Mountain, though one song was written in dropped-A# because I happened to be tuned in C-standard and needed a drop tuning to finger the chords properly (no, not power chords, I'm not that poor of a musician).
As far as everything else goes, I'm usually in C-standard tuning, though I will switch it on occasion if I'm playing along with another song and I don't feel like playing along to Guitar Pro.
There's really no reason for me tuning to C-standard as opposed to say, D-standard, it's just what I'm used to. E-standard, however, is too high, IMO, for heavier stuff.