True, your bag may be last off, but it does mean that the chance of it actually being there is better. I have been told that it takes about 15 minutes for a bag to be processed through the sortage system, then it has to be delivered to the aircraft which can take some time if you are leaving in the morning "rush hour" at Gatwick. If you leave it until the last minute to put your bag in you are tempting fate a bit. This is one of the reasons that BA brought their check-in closing time back from 30 mins to 45.