Just a general question about rear rotors ... changing rear pads on 2006 Outback wagon (pad indicator squeling so it's time) .. inspected rotors good shape no signs of warping .. took out a set of calipers to measure rotor thickness in each quarter  --> .390" - .400". Do you ever have to change rear rotors, will you even detect "runout" on them and if so what is min spec is for rear rotors ?   

I don't know the numbers off hand, but it's very unlikely that they would ever need to be changed unless they were run with a completely ruined pad (metal on metal).

Discard from Subaru on all EJ cars with solid rear disc rotors is .335". Unless you have a serious pulsation in the brake pedal or are heavily rusted, I would merely resurface them and reuse them.

Run them.  Probably done it once and forgot but i've never replaced Subaru rear rotors. 200,000+ miles is common for them and they're not even getting a work out with daily driver duties.