Any system that is designed to be placed on its side will have the rubber feet attaching to the support side panel.  While you still can operate on its side, take into consideration of other risk of cosmetic damages beside cooling.  Just like when opening up a computer, obvious orientation will be keeping the removeable panel side up and placing motherboard side on the bottom.
I have a Dell G5 5000 all original stock parts that I have had on a table (case vertical) since I purchased it a few years ago.  Thinking about moving furniture around which would require putting the case on its side.  The only vents are front and back so it seems okay from that standpoint but not sure if the case needs to be upright so that airflow works as designed. Any thoughts are appreciated!  
Does Dell G5 5000 Case Orientation Impact Cooling?