By my reading, this was the OP's question. And I tend to agree with your advice. I know some posters report that availability tends to open up quite close to flight departure, especially in premium cabins, but I personally would never feel comfortable taking that risk. My approach in these situations has always been that either I find availability for the roundtrip right from the beginning, or don't use miles at all. At least if OP has confirmed a seat in a lower cabin he knows he'll be able to fly that day, but still it ends up being an expensive mileage flight at the J rate.