Biological Filtration?

[Crosby]

I will be upgrading from my 90 gallon to 180 gallon. My 90 gallon used a wet/dry with bio-balls. For my 180 I want to stay away from bio-balls. I will have 250 pounds of live rock in the main display tank that will be my biological filtration with a 1 inch sand bed for asthetics. I have seen pictures of tanks that have 100's of pounds of live rock in the main display and also have live rock (rubble) in their sumps. Do I need to also add liverock to my sump and if so for what purpose?