No expert, but I think the cans have a mark or indicator for the best locaiton and direction to spray from the can. I believe it means the pickup tube is toward that side so when you are painting with the can at an angle the tube is still in the paint if it is not
still well shaken. If you lay it on its side put it with that side down especially if the can is partially empty so the tube remains in the paint after that settles rather than up in just the propellant where it might dry out. Just a theory,