as an interesting note, and you can try this out for yourself up at a safe altitude....if your aux tanks are low (not sure at which point exactly, but let's say half or less in them) and you are in descent, they will 'unport' and its as if you've run out of gas. But you can climb on low aux tanks all you like.
don't ask me how I know all this for sure, but just know it will be an uncomfortable moment if you are in descent for landing and suddenly your engine shuts off because you forgot to switch back to mains as part of the landing checklist....especially if it happens to you more than once
