The floors you would usually rush in dungeoneering, you do as complexity two. At the start you buy feathers and a rod, and fish every spot you see. They nerfed the xp a little while ago, so I don't believe it's efficient to do if you don't care about dungeoneering, (and if it is, then it's only at 90+ fish) But if you train dungeoneering and would spend time doing the same floors in complexity one anyway, then getting the extra fishing xp would be a good idea. Obviously, the method becomes better at ever tenth fishing level, as you only get fishing spots from the two highest fish you can catch. (at level 80-89, you only get salve eel and blue crab fishing spots)