Do not use an impact to fully tighten the nut, but it is OK to use one to lightly thread it on. The reasoning behind the warning is to minimize the possibility of damage to the strut rod.
What I usually do is use the impact to lightly tighten the nut (while the assembly is still on the spring compressor), then reinstall the entire assembly into the vehicle and use a torque wrench to torque the nut to spec. Once the assembly is back on the car, the rod will not move much at all and you will be able to retorque the nut. This is actually the method Toyota recommends.