[wilkin250r]

The absolute worst-case scenario is if the return-line valve goes bad but the feed-line valve is still working, you dump your return-line fuel into the opposite tank. No problem if that tank is empty, but it could become a problem if that tank is full.

I would imagine the VAST majority of malfuctioning valves get stuck in the default position, not in the activated position. So I would make a habit of making the default tank your "main" tank, and drain it first. That way, on the small 0.001% chance you ever have a malfunctioning return-line valve, it will be while your second tank is full, and your main tank is empty, and your return line is dumping into your empty main tank.