Originally Posted by Back Road Runner
Just be careful. On the stock strut, it's ok. On aftermarket ones with adjusters, the shaft is hollow, and you can warp it and cause problems. For example, Tokico specifically states not to use an impact gun on their D-Spec struts because of this. You can still get away using one, but you really have to be light with it and sort of do it "just enough."
Just use it at the lowest setting, install them in the car, then torque them. I've always used an impact gun on struts and never had a problem but I run at a low power level.