Rank 50 allows you to have more output, which in turn allows you to have more attempts at grade 5 nodes and below. Also, grade 10 nodes are the best to fish at (Isles of Umbra) because the casts are so easy, you are guaranteed around 300 sp, and practically a guaranteed catch since the fish are only grade 1. With sweat of brow (Rank 25 botany skill), I'd say leveling Fishing to 50 is quick, easy, and profitable. But...that's just me, and I'm working on all my gathering to 50...so....think what you want!