Never used a rail, did consider getting one when we turned the cot into a toddler bed but it seemed most kids could manage without so I didn't see the point in the end. I did read that a 'bed safari' was worth while though - this is where you spend lots of time in the day playing on and around the bed, walking around, crawling on the bed (right around the perimeter), crawling on and off etc etc, the more you do the more the mental map of the size and shape is built, this is the map that is used during sleep to keep LO in there.
I put a few spare pillows and a duvet on the floor either side but he only fell out about 4 times, he was much younger.