I agree with REE. When you're separating the flies it is just so much easier to divide them equally. I always send a fly back, and expect it myself when I send out a swap box. It makes for an easy comparison as a tyer, but for the swapmeister, going through to make sure you have the right fly in the right box . . . I just make 12 piles and put a fly from each box in a pile, put 'em back in the boxes. You can't imagine how many times we've sat here counting flies, comparing who sent what, which fly is eliminated from which box, and so forth. Anymore it's just 12 piles, 12 flies, 11 boxes (and my generous pile!).
And the extra fly is a good thanks for the organizer. Some tyers send more than one, and sometimes even flies that are different than the swap. That's always fun!
For those who have either never entered a swap, nor hosted one, this is just a kick. Especially hosting one. I love to see how different people package and to witness the conformity of the dozen flies. This has helped my tying and made me more aware of what comes off my vise. I just finished sending out a deer hair swap I hosted here, expecting to see a whole bunch of mice and big hairy poppers only to have some of the most beautiful small trout flies come in, many with some incredible craftmanship involved. As a warmwater tyer, this just completely opened up a whole new world of tying ideas I didn't realize even existed. Before I swapped I just enjoyed the physical and creative side of tying. Swaps have expanded that immensely. What a great way to add to a hobby. JGW