In my youth (a very long time ago now!), the rule of thumb was 3 times the rod test curve was the lightest line you could use, 5x was the ideal and 7x was the heaviest so that for a 1lb TC, the minimum recommended line strength was 3lb bs, the ideal 5lb and the heaviest 7lb bs.
However, that was in the days of fibre glass rods and lines of inferior quality to today. I'm sure that with the modern lines and the forgiving nature of the best carbon rods these days, you can get away with lighter/heavier lines but the key point is that you have to adapt your 'playing' approach to the tackle and obviously don't try to bully fish on light lines or play fish too hard on stronger tackle..........
Dave