You can set following values to 0 to get an exact 0 sec casting time.
Art - Animation Cast Point
This is when your unit starts the effect of an ability.
Setting this to 0 will make your unit's spell to take effect immediately after you hit the button; but if are using animations on other spells -on same hero-, this will make them look weird =) (like damaging first, then playing the animation =))
Art - Animation Cast Backswing
This is when your unit finishes the effect of an ability.
Like in dota, when you cast frost nova with maiden; it finishes casting the spell but the icon is still glowing -showing that spell is still in casting state- and hero stands still, not attacking nearby enemies.. Well, reducing this value is a good way to get rid of that.
If you want preventing order interuptions upon casting; you can use non-order-interuptive skills like mentioned in the previous post =) And trigger the real spell effects via "triggers"