Do you recommend initiating an ACE inhibitor or ARB in a patient with proteinuric CKD Stage 5?  

I would if the potassium is less than 4.8. I would repeat potassium in 2 weeks after starting to make sure it has not gone up. If it is more than 5 after starting then would stop. Of course, if the eGFR is really low or the patient is symptomatic will start dialysis and then consider starting ACE/AR...