Do I need to get new lug nuts to use tire-rack steelies on an '06 outback that has aluminum-rim lug nuts?

The closed end "Acorn" style nuts might bottom out on the stud before getting to the wheel.

Remove a wheel and spin one of the lug nuts all the way down on the stud and see how much room there is between the cone of the nut and the face of the brake rotor. More than 1/8 of an inch you probably want to get different lug nuts. I'd stick with a closed end style (to keep rust out) but longer.

It should be fine.

 

If you're buying them new from tire rack, they include a set of bulge-seat acorn nuts, but the contact patch looks the same.