It doesnt really matter if you pay attention to everything. The only difference laying something flat vs verticle vs slanted or what ever is gravity right?
If gravity is effecting your paint job, you have much bigger problems to worry about! IE, runs, sags, drips. When your gun is set right and the materials are mixed properly for the tempature in the booth, thickness, etc, your basecoats will flash quickly. If you have metallics or pearls, and those effects are sagging, puddleing up, or flowing around on the part then you need to review your mixtures, gun settings, or gun movements.
That door was painted front and back one shot. Its alot easier to shoot the underside that way than working a gun around 4 mounting posts and the stand itself if it were hung on our stand.
Lastly, coverage. You dont need to put the doors and bumpers on the car to paint them and have them match if your not blending. Those were easy, becasue it was a black primer and a black base. so just count your coats and keep each one consistent on the overlaps and coverage. If it were a big contrast difference between primer and base, its alot harder… but thats why they make tint’s for primers. 
