Rhys - think most people seem to be concerned about a car being strapped down to stop it going sideways or forwards off the rollers rather than the fact of accurate torque figures (it shouldn't go forwards as the car is stationary and there is no inertia in the car). If a car trys jumping out of the rollers then it obviously isn't an accurate figure - strapping the car down and/or sitting on it would help stop this, so i suppose yes it does. But if the car isn't jumping out anyway then probably doesn't affect it.

Think you might open a can of worms with this one though...