I have a drawer in my filing cabinet that I use for both project and general reference folders (my system isn't that big yet). I like straight line filing because I can't be bothered to decide which type of tab is next, but also because I have assigned meaning to them: left 1/3 cut tabs are for projects, center tabs are for general reference, and the right tabs I just fold the other way so that they become left tabs. Completed project folders get turned around so that their tabs are on the right side, at the back. When my system gets bigger I'll put the completed project folders in a separate drawer.