Yea, the upload gets really tricky. You really just want to do some real life test of your upload and set it to that. Thats what the "Auto" setting "tries" to do. It does a capacity test to see what the max transfer rate and sets it to that. The problem being most Cable ISP use some soft of Speedboost, or Powerboost, to increase upload and download speeds. Not only does this make your upload inconsistent, but the act of using powerboost can actually change the way the data is piped and add latency to it.
So yea, way to go is test, set, then trial and error. First see what you "should" get, not including powerboost or anything. Then run some test, to several servers, at different parts of the day. If you get more then your standard rate, then set it to your standard rate. If you get less, then take the adverage and set it to that (but if you get less then 50% of your advertised speed, contact your ISP and make sure everything is OK). Once thats done, try it out and see if theres a difference. To be honest, I really think it doesn't matter all that much, if at all. Let me know though.