Think we leave ours up
The book that came with us shows the use of each position. When in car it's down against the seat. On pram its up. When placed on floor it's behind the seat leaning on floor to stabilise.
It may depend on the car seat but for mine (Maxi Cosi Pebble) the user manual says keep it up.
I’ve always left mine up. In my mind, it acts as a roll cage x