You can print upside down just fine. "leveling" here is defined as, after homing, setting the distance to the nozzle and the bed and also making sure the distance is the same everywhere on the bed.
For example these backpack ultimakers print just as good as ones on a table:
The bed needs to be 'level' in respect to the print nozzle. More specifically, it needs to be parallel to x and y axis gantry guide rods.
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