Modern AF Long Tele lenses should be okay stored in any direction but if you are into older manual focus stuff like me, you might want to invest in tall cabinets or Pelicans and Silica Gel. Older lenses have tight helicoids that are greased and unless you rotate them every now and then, the lubricant actually settles and leaks out the side of the lens.
Hope this is helpful, GOD Bless.