It does not put you at any higher risk of offloading since they ask for volunteers if the flight is oversold and DL is pretty much willing to go as high as they need to ensure there is no involuntary denied boarding. As for seats, you have to make the calculation of the likelihood of getting a better seat or worse seat if you wait until its automatically assigned. There are usually a group of seats blocked off at the rear of the aircraft, plus a couple more throughout. The seats at the back are usually assigned to groups that are travelling together that also don't have seat assignments, but if there aren't any groups, then you might end up in those seats (might get and aisle or a window) which may be better than what you have right now if you are stuck in a middle seat. On the other hand, depending on the aircraft and route, you might have a lot of "preferred" seats that can be reserved by medallions or C+ seats that are available, and if there aren't enough medallions to fill those seats and there are not enough coach seats to accommodate everyone with a Y ticket, you might luck out and get moved up.