How would you counsel non-cardiologists needing to prescribe a QTc prolonging medication in a patient with an EKG showing prolonged QTc but no history of long QTc syndrome and no known cardiomyopathy?  

This comes up frequently in oncology where antiemetics may be avoided due to seeing this on an ECG done, often inpatient when deciding whether or not to start these medications.  What arrythmia risk risk does this carry in the absence of any structural heart abnormality?