Not nesesarily, the laws of physics that apply to fly casting have to do with a match in the physical weight of the fly line that is being cast and the physical weight of the fly attached to that line as they need to travel at the same speed. If there is to much of a weight difference the line and fly will travel at different speeds resulting in a poor cast. Large fish may be handled relatively easily on lighter rods although there are some reasonable guidelines, especially if you intend to release the fish unharmed. A 5 weight rod will handle most 7 to 10 lb steelhead just fine.