you mean gravel or big old rocks? gravel=nothing itll be ok... big old rocks= make sure there 110% sturdy and theyll be fine, BUT if they fall well when a rock falls on glass what happens?
Most hardware or electrical supply stores will have something called "eggcrate". It's usually used as a light diffusor for fluorescent lighting, but many fishkeepers use it for rock weight distribution as well...
Cut it to the correct size for your tank and place it on the bottom. The rocks go on it and the weight is distributed. You should add the sand afterward, especially if you're getting cichlids that like to dig, as many do.