I used to go the "only the materials I need" route, but now I carry as much as I can, albeit in small quantities. Just this year I was ambushed by an unexpected stonefly hatch and helgrammites in a stream where they weren't supposed to be. Guess what the fish were feeding on. The nearest fly shop was three counties away, my tying bag was in the truck. If I hadn't brought the extra stuff I'd have been blanked, as it was I had what I needed and we caught some really nice fish all week. Flying of course is a different story with what airlines are charging for luggage. In those cases I just carry every fly I own, and don't worry as much about tying.