Most single-prop pilots are keenly aware of the veering to the left when starting the take-off run. The prop-wash effect is clearly understood and well simulated in X-Plane.
But the problem of not flying straight when in flight is a different matter. For all those who are agonizing about how to prevent a Cessna 172 from veering off to the left or right when cruising, there is a very important recent thread on X-Plane.org with loads of good information about this matter. Take particular note of @skycycle’s post on the subject.