There really isn't much of an answer for it.. Depends on the pitcher, and how well you can control your curve, relative to your FB and anything else you may throw
You can mix it up, and throw a first pitch curve, or throw it 2-1 to give someone a different look
Throw it as a strikeout pitch, or be selfish and go with a high heat..
You can throw it any count if you can control it
But my suggestion is
0-0 =to show something other then a first pitch FD
and anything ahead 0-2, 1-2...