You need to back out in both cases, literally zero difference whether you do it when arriving or leaving makes no difference, but loading your grocery when park

You need to back out in both cases, literally zero difference whether you do it when arriving or leaving makes no difference, but loading your grocery when parked head in is way more convenient than the other way in which case it can be striaght up impossible with car parked against you or the wall in underground garage and narrow spaces between cars where you can't fit shopping cart.