Rocks are useful for cooking in panel cookers, for stabilization in wind, propping panels, using under pots in place of a grate (pebbles), etc. In box ovens they can provide thermal mass and can also be used under the front of your bottom tray to make it more level, if you tip your cooker to get more out of low sun. Aside from the pebbles to be used in place of a grate, an assortment of rocks in the 1-3 fist-size range is very handy to have, and a few that are even bigger if you need to stabilize the base for a big funnel or windshield cover cooker.
If you will be using rocks where they would cast shadows on your reflectors, wrap the rocks or pebbles in foil, so they willl throw light instead. For thermal mass in an oven, dark rocks are best, but you can also give them a light coat of paint to darken them.