Backing is important and every so often you should replace it. Just place enough backing on the reel so your fly line almost totally fills the reel. Too much backing and the fly line will hit the foot section of the reel. For most trout fishing it does just fill the reel, but, it keeps your line stored in a large a circle as possible on the reel, hence decreasing the coil memory of the line, thus you get better casting.
Larry ---sagefisher---
Organizations and clubs I belong to:
Fly Fishers International Life Member
FFI 1000 Stewards member
FFI Presidents Club
FFI Fly Tying Group Life Member
Washington State Council FFI
V.P. Membership
Alpine Fly Fishers Club
President & Newsletter Editor--The Dead Drift
North Idaho Fly Casters club