The primary reason for changing the velocity curve is to adjust for the particular keyboard/piano which is to be used with the software. Therefore, no general velocity curve exists. What that means is that if you would like to use the velocity curve from someone else, then that person should have the same hardware (piano/keyboard, or at least the same action) that you have.
But even the curve from someone else using the same hardware may not suit you perfectly - or at all, because it also depends on playing style. So I would certainly encourage you to play a bit around with the velocity curve, to see what kinds of changes work well for you. Maybe the straight line actually works best.
It could be said that there are two fundamental types of changes from the straight line: 1) one that bends upwards (i.e. it is above the straight line), and 2) one that bends downwards (i.e. is below the straight line). It should be fairly easy to get a feel for which of the two types of changes makes things better, and which makes it worse. They may both make it worse. 🙂
