Never vertical, only flat and on bearers.
The big DIY sheds always store vertical for logistical reasons, they dont care if you have trouble fitting a twisted door.
But you need to make sure the floor or bearers are really flat. To do that ideally lay three 900mm bearers, 100mm x 50mm lets say, on the floor one about 300mm down from top of the door, another in the centre line and the third about 300 up from bottom of the door.
Now the tricky bit, we need to check that the bearer tops are all in line with one and other. To do this lay on the floor at one end with your head almost on the floor, either top or bottom doesn't matter as long as you can only see the 1st bearer, raise your head so as your sight line comes up the top edges of the 2nd and third bearers come into view until all three bearer tops are in line. Without moving your head eye across the width of the bearers to see that they are all parallel. If they are not you can pack up the end of the one that is low with card or folded paper. Keep doing this until you are sure they are all in line.
Carefully place the first door on the bearers and stack others on top. you can use spacers between the doors but that is only really necessary if you are wanting to acclimatize them to the building.
Cover to keep light and dirt off the stack, and put weight on the corners and middle of the top door just in case it wants to twist out of shape.
They can stay like that indefinitely.
Steve.