Negative. If you roll exactly what you need, it's a crit. So if your effective skill is 18, and you roll 18, it's a crit.
As an aside, crits are treated as if you needed and rolled a 20 for competing rolls.
So if you need <=18 and roll 17, but your opponent needs <=12 and rolls 12, to determine who won, it is as if the opponent needed and rolled a 20, so he would win.