How long is the flight. If she asked nicely and the flight was 2 hours or less. I probably would have moved my bag. If its a transcon, or they acted like a ....... I would have said no. I generally only put my backpack up later after its becoming apparent that the flight isn't full, or its a 6hr plus flight and at that point whats another 30 minutes at baggage claim.

The only reason I dont typically put my bag up is because of one particular bad experience. Because of delays I had a very tight connection, I didn't get my typically zone 2 boarding, and boarded with group 8 or something, All of the economy plus bins were full of backpacks, and 80% of those passengers had nothing under seat and I had to check my roller board. Now some of the very large backpacks I get, but some of these were moderate regular sized bags. Also it was a 2:15 minute flight. So now im much more conscious of putting my backpack up there.

Im also not a very tall guy. My brother is 6'4", He never puts anything under his seat.