Usually closed. Work PC used to be open for accessibility. Now that its in a nice case its closed. Helps with noise too! I do have two test benches mounted vertically though. Just motherboard trays on a board.
Benchtable type of cases are by far the best for air cooling, and you don't need any additional fans.
I've been through many types of cases, water cooling, but ended up going back to open air case (dimastech).
It is simple and effective, not only shows whole rig better than any case with even the best window, but provides great cooling with minimal amount of fans.
Occasionally leave the side panel off, live in the desert, it gets hot here. But mostly leave it off when tinkering with the insides, swapping hard drives, video cards, etc. Otherwise too much dust and the panels are on.
I honestly only ever have my side case off when I am testing, cleaning, or trying to troubleshoot any possible problems. Otherwise it is pointless IMO to have the fans you have in there if you do not have a good flow of air. Leaving your case open, if you use a case, would defeat the purpose of using the case IMO.