For prostate cancer, when do you pursue a biopsy for metastatic disease to the bone found on imaging study?

Presuming the patient has undergone curative care for his prostate cancer, and had become undetectable via PSA, who now presents with bone metastases, when would I consider a bone biopsy?

So, if he was treated for cure, and had an undetectable PSA on his most recent follow up visit, my question beco...