Stumpy tails is another one of those tough things to judge. The now 9 week old pullet that survived the dog attack had a stumpy tail at 4 weeks and I swore that she was a cockerel. But, she's not. I'd say that all of the things we are talking about, wing feathering, feathering in general, wattles and tail feathers are all useful for sexing about 90% of the time. It's that 10% that really stump me!!! Anyway, if all of the signs are there, then I can usually say it is a cockerel with some accuracy. It's when only one or two of the signs are there that I just decide to wait and see. I've read too many posts where they thought it was a he and at about 8-10 weeks realized it was a she and visa versa.
By the way, at 2 weeks I think all I hatched were cockerels, at 3 weeks I begin to think they are all pullets, at 4 weeks I can usually sex about 1/2 of my chicks accurately, at 6 weeks I can sex a few more and by 8 weeks I usually have almost all of them sexed. There are always one or two that I have to wait the full 10 weeks on before I can sex them. I have one right now at 6 weeks old that I'm leaning cockerel, but I still don't have any definite signs that it is one or the other except for slow feathering, but it is also the runt so it could just be slow to mature.
Anyway, I can't wait until we all find out how accurate our guesses are!