A friend who knows his stuff advised me to lay them flat so at my old house I stored about a dozen on the top shelf of some industrial shelving (the kind that has good, stiff wooden shelves, not the ones that use wire shelves.) I used a combination of tissue paper and butcher's paper to keep them separate from each other with the expectation that I could polish out any fine scratches the paper caused. Most had been clearcoated and were able to offgas just fine this way. They were up there for over seven years with zero issues.

Unfortunately at my new place I don't have the ability to do that so I now have them stored vertically in playfield boxes which are stuffed with playfields so no possibility of them leaning at an angle inside the box. It's been four years and the last time I pulled one out it was still just fine.