Good topic. You have come up with perhaps some decision rules yourself.....teams know them already. Saying that you should NEVER trade up is a huge leap though.
Do you need a QB in the NFL? Of course and the record shows that moving up into the TOP 5 is the best way to get one. On occasion you might get very lucky in the first 2-3 rounds or with Brady later. But, in the last 10 years name another QB who was picked after the 75th pick (Wilson)? Another decision rule may be you only do it for a QB or a franchise changing player who is that one player you need.
As for the risk management argument, if you trade back enough you minimize your risk but end up with all 7th round picks. At some point you need to get some skin in the game and try to get that guy. Guaranteed success? No, but it doesn't send your team to bankruptcy and kill it. It sets you back perhaps and you move on.
You can hedge your bet by taking a second QB a year or two later in the first 3 rounds.