It is semantics but nonetheless important - you really meant to say never transfer MR -> miles until you are holding "a confirmed reservation". You will never get a "ticket" (paper, electronic or otherwise) until the miles are deducted from your account.

I tried to use CO miles to upgrade my NW far to BUH. I was told on the phone all was in order, transfer the miles. I did the transfer, they appeared immediately in my CO account. Then CO came back and said "NW said no upgrades." By this time I had been back and forth on the phone for 2 hours and was not ready to keep pursuing it. So I had 20,000 miles in CO that I really didn't need to transfer there. I will eventually use them, but even with the best intentions, mistakes happen.