When do you screen for leptomeningeal involvement in myeloma?

When a plasmacytoma of the calvarium, orbit, skull base, or spine is in immediate proximity to the CNS, especially if there is mass effect on the brain or cord, I worry about invasion across the blood/brain barrier. In a few cases I have performed a lumbar puncture to screen the CSF. Is this justified, or should it be reserved for cases where there is leptomeningeal enhancement on MRI?