I've never done a big wall, or any climb for that matter, that employed fixed lines. When fixing a line what exactly is the rope fixed to? Bolts I'd imagine or a natural feature.
Any solid anchor. It would depend on the route and situation as to whether is was bolts, pro, or both. The fixed line us just a means to an end (getting back up to the high point). Any safe and efficient way you can figure out how to do that is what you want. Some consider it better style not to fix lines to the ground. ie. once you leave the ground you stay on the rock till it's over.
Always remeber to attach the loose in of the fixed line to a bottom anchor, so it doesn't blow around on the rock face and get snagged. I've heard horror stories from this happening on El Cap.