Over time, leaf springs will straighten out and coils will get closer together. Taking the load off will help reduce this and extend the time the springs retain their original designed positions. Whether this is done with jacks or blocks makes no difference other than with the jacks down the rams are exposed to the elements and need to be protected with teflon, silicon or ship-2-shore spray, grease will work but messy.
'98 Southwind 34L, too many cameras, old hat, young heart.
May the light be with you.