Filling audio rack tubes with sand, metal, grape nutz???

Hello-

I know this has been asked before, but can't seem to find the answer I need. I have to VTI pro series audio racks coming this Friday from Cali and was wondering if it's necessary to fill the tubes with anything. I've researched people use sand or metal pieces of something or another. Any recommendations would be great unless it's Grape Nutz. 

Thanks!

Brent