There are two schools of thought on this.
you should pack everything on the main level first,
and proceed to the upper or lower levels.
This will allow you to clear space on the main level for boxes and materials on the levels.
using this method when the delivery truck is loaded,
all the main level items will go on first.
this will result in easier unloading to the other levels when the move is completed.
The other school of thought is to pack items that are not regularly needed first.
Then when the truck is unloaded the items that are needed will come off the truck first.