Do you routinely prescribe PPI prophylaxis for myeloma patients who will be on low-dose aspirin and weekly dexamethasone?  

Depends on the dose of weekly dexamethasone. If 40 mg weekly, would use PPI. If given along with cyclophosphamide as CyBorD, would use PPI regardless of dose of dex; if symptomatic gastritis on aspirin or dex, will use PPI even with lower doses of dex.