Of course if you want the highest possible water flow to the filter intake you wouldn't have any decorations at all in the tank
As TankGirl said above you have to strike a balance between efficient waterflow and what looks good to the eye. Look at your tank and see if there is any debris gathering in spots on the bottom. If so, you might want to rearrange your decorations. You might even consider adding a powerhead to increase water circulation.
Gene Heitman - 12 tanks (11 freshwater and 1 saltwater), 206 gallons, 20+ species of fish/shrimp/snails, 52+ species of plants ... 10 years ago I just wanted 1 tank with some fish